The "pane" of glass, so impossibly thin that its individual silicon and oxygen atoms are clearly visible via electron microscopy, was identified in the lab of David A. Muller, professor of applied and engineering physics and director of the Kavli Institute at Cornell for Nanoscale Science.

The work that describes direct imaging of this thin glass was first published in January 2012 in Nano Letters, and the Guinness records officials took note. The record will now be published in the Guinness World Records 2014 Edition.

Just two atoms in thickness, the glass was an accidental discovery, Muller said. The scientists had been making graphene, a two-dimensional sheet of carbon atoms in a chicken wire crystal formation, on copper foils in a quartz furnace. They noticed some "muck" on the graphene, and upon further inspection, found it to be composed of the elements of everyday glass, silicon and oxygen.

They concluded that an air leak had caused the copper to react with the quartz, also made of silicon and oxygen. This produced the glass layer on the would-be pure graphene.Besides its sheer novelty, Muller said, the work answers an 80-year-old question about the fundamental structure of glass. Scientists, with no way to directly see it, had struggled to understand it: it behaves like a solid, but was thought to look more like a liquid. Now, the Cornell scientists have produced a picture of individual atoms of glass, and they found that it strikingly resembles a diagram drawn in 1932 by W.H. Zachariasen -- a longstanding theoretical representation of the arrangement of atoms in glass.

"This is the work that, when I look back at my career, I will be most proud of," Muller said. "It's the first time that anyone has been able to see the arrangement of atoms in a glass."

What's more, two-dimensional glass could someday find a use in transistors, by providing a defect-free, ultra-thin material that could improve the performance of processors in computers and smartphones.

Scientists at the Universities of Oxford, St Andrews, Bristol and the Max Plank Institute in Nijmegen, the Netherlands, found correlations between handedness and a network of genes involved in establishing left-right asymmetry in developing embryos.

'The genes are involved in the biological process through which an early embryo moves on from being a round ball of cells and becomes a growing organism with an established left and right side,' explained first author William Brandler, a PhD student in the MRC Functional Genomics Unit at Oxford University.

The researchers suggest that the genes may also help establish left-right differences in the brain, which in turn influences handedness.

They report their findings in the open-access journal PLOS Genetics.

Humans are the only species to show such a strong bias in handedness, with around 90% of people being right-handed. The cause of this bias remains largely a mystery.

The researchers, led by Dr Silvia Paracchini at the University of St Andrews, were interested in understanding which genes might have an influence on handedness, in order to gain an insight into the causes and evolution of handedness.

The team carried out a genome-wide association study to identify any common gene variants that might correlate with which hand people prefer using.

The most strongly associated, statistically significant variant with handedness is located in the gene PCSK6, which is involved in the early establishment of left and right in the growing embryo.

The researchers then made full use of knowledge from previous studies of what PCSK6 and similar genes do in mice to reveal more about the biological processes involved.

Disrupting PCSK6 in mice causes 'left-right asymmetry' defects, such as abnormal positioning of organs in the body. They might have a heart and stomach on the right and their liver on the left, for example.

The researchers found that variants in other genes known to cause left-right defects when disrupted in mice were more likely to be associated with relative hand skill than you would expect by chance.

While the team has identified a role for genes involved in establishing left from right in embryo development, William Brandler cautioned that these results do not completely explain the variation in handedness seen among humans. He said: 'As with all aspects of human behaviour, nature and nurture go hand-in-hand. The development of handedness derives from a mixture of genes, environment, and cultural pressure to conform to right-handedness.'

Super-Earths are emerging as a new type of exoplanet (i.e., a planet orbiting a star outside of our Solar System) with a mass and radius larger than Earth's but less than those of ice giants in our Solar System, such as Uranus or Neptune. Whether super-Earths are more like a "large Earth" or a "small Uranus" is unknown, since scientists have yet to determine their detailed properties. The current Japanese research team of astronomers and planetary scientists focused their efforts on investigating the atmospheric features of one super-Earth, GJ 1214 b, which is located 40 light years from Earth in the constellation Ophiuchus, northwest of the center of our Milky Way galaxy. This planet is one of the well-known super-Earths discovered by Charbonneau et. al. (2009) in the MEarth Project, which focuses on finding habitable planets around nearby small stars. The current team's research examined features of light scattering of GJ 1214 b's transit around its star.

Current theory posits that a planet develops in a disk of dense gas surrounding a newly formed star (i.e., a protoplanetary disk). The element hydrogen is a major component of a protoplanetary disk, and water ice is abundant in an outer region beyond a so-called "snow line." Findings about where super-Earths have formed and how they have migrated to their current orbits point to the prediction that hydrogen or water vapor is a major atmospheric component of a super-Earth. If scientists can determine the major atmospheric component of a super-Earth, they can then infer the planet's birthplace and formation history.

Planetary transits enable scientists to investigate changes in the wavelength in the brightness of the star (i.e., transit depth), which indicate the planet's atmospheric composition. Strong Rayleigh scattering in the optical wavelength is powerful evidence for a hydrogen-dominated atmosphere. Rayleigh scattering occurs when light particles scatter in a medium without a change in wavelength. Such scattering strongly depends on wavelength and enhances short wavelengths; it causes greater transit depth in the blue rather than in the red wavelength.

The current team used the two optical cameras Suprime-Cam and FOCAS on the Subaru Telescope fitted with a blue transmission filter to search for the Rayleigh scattering feature of GJ 1214 b's atmosphere. This planetary system's very faint host star in blue light poses a challenge for researchers seeking to determine whether or not the planet's atmosphere has strong Rayleigh scattering. The large, powerful light-collecting 8.2 m mirror of the Subaru Telescope allowed the team to achieve the highest-ever sensitivity in the bluest region.

The team's observations showed that GJ 1214 b's atmosphere does not display strong Rayleigh scattering. This finding implies that the planet has a water-rich or a hydrogen-dominated atmosphere with extensive clouds.

Although the team did not completely discount the possibility of a hydrogen-dominated atmosphere, the new observational result combined with findings from previous research in other colors suggests that GJ 1214 b is likely to have a water-rich atmosphere. The team plans to conduct follow-up observations in the near future to reinforce their conclusion.

Although there are only a small number of super-Earths that scientists can observe in the sky now, this situation will dramatically change when the Transiting Exoplanet Survey Satellite (TESS) begins its whole sky survey of small transiting exoplanets in our solar neighborhood. When new targets become available, scientists can study the atmospheres of many super-Earths with the Subaru Telescope and next generation, large telescopes such as the Thirty Meter Telescope (TMT). Such observations will allow scientists to learn even more about the nature of various super-Earths.

The scientists investigated the genomic basis for echolocation, one of the most well-known examples of convergent evolution to examine the frequency of the process at a genomic level.


Echolocation is a complex physical trait that involves the production, reception and auditory processing of ultrasonic pulses for detecting unseen obstacles or tracking down prey, and has evolved separately in different groups of bats and cetaceans (including dolphins).The scientists carried out one of the largest genome-wide surveys of its type to discover the extent to which convergent evolution of a physical feature involves the same genes.

They compared genomic sequences of 22 mammals, including the genomes of bats and dolphins, which independently evolved echolocation, and found genetic signatures consistent with convergence in nearly 200 different genomic regions concentrated in several 'hearing genes'.To perform the analysis, the team had to sift through millions of letters of genetic code using a computer program developed to calculate the probability of convergent changes occurring by chance, so they could reliably identify 'odd-man-out' genes.

They used a supercomputer at Queen Mary's School of Physics and Astronomy (GridPP High Throughput Cluster) to carry out the survey.Consistent with an involvement in echolocation, signs of convergence among bats and the bottlenose dolphin were seen in many genes previously implicated in hearing or deafness.

"We had expected to find identical changes in maybe a dozen or so genes but to see nearly 200 is incredible," explains Dr Joe Parker, from Queen Mary's School of Biological and Chemical Sciences and first author on the paper."We know natural selection is a potent driver of gene sequence evolution, but identifying so many examples where it produces nearly identical results in the genetic sequences of totally unrelated animals is astonishing."

Dr. Georgia Tsagkogeorga, who undertook the assembly of the new genome data for this study, added: "We found that molecular signals of convergence were widespread, and were seen in many genes across the genome. It greatly adds to our understanding of genome evolution."Group leader, Dr Stephen Rossiter, said: "These results could be the tip of the iceberg. As the genomes of more species are sequenced and studied, we may well see other striking cases of convergent adaptations being driven by identical genetic changes."

1918 October - Arab troops led by Emir Feisal, and supported by British forces, capture Damascus, ending 400 years of Ottoman rule.

The Ottoman governor of Syria, Jamal Pasha, rides through Damascus in 1917

1919 - Emir Feisal backs Arab self-rule at the Versailles peace conference, following the defeat of Germany and the Ottoman Empire in World War I.

1919 June - Elections for a Syrian National Congress are held. The new assembly includes delegates from Palestine.

1920 March - The National Congress proclaims Emir Feisal king of Syria "in its natural boundaries" from the Taurus mountains in Turkey to the Sinai desert in Egypt.

French control

1920 June - San Remo conference splits up Feisal's newly-created Arab kingdom by placing Syria-Lebanon under a French mandate, and Palestine under British control.

1920 July - French forces occupy Damascus, forcing Feisal to flee abroad.

Aleppo, along with Damascus, is one of the oldest continuously inhabited cities

1920 August - France proclaims a new state of Greater Lebanon.

1922 - Syria is divided into three autonomous regions by the French, with separate areas for the Alawis on the coast and the Druze in the south.

Uprising

1925-6 - Nationalist agitation against French rule develops into a national uprising. French forces bombard Damascus.

1928 - Elections held for a constituent assembly, which drafts a constitution for Syria. French High Commissioner rejects the proposals, sparking nationalist protests.

1936 - France agrees to Syrian independence in principle but signs an agreement maintaining French military and economic dominance.

1940 - World War II: Syria comes under the control of the Axis powers after France falls to German forces.

1941 - British and Free French troops occupy Syria. General De Gaulle promises to end the French mandate.

1945 - Protests over the slow pace of French withdrawal.

1946 - Last French troops leave Syria.

Baath Party founded

1947 - Michel Aflaq and Salah-al-Din al-Bitar found the Arab Socialist Baath Party.

1949 - Army officer Adib al-Shishakhli seizes power in the third military coup in the space of a year.

1952 - Al-Shishakli dissolves all political parties.

1954 - Army officers lead a coup against Al-Shishakli, but return a civilian government to power.

1955 - Veteran nationalist Shukri al-Quwatli is elected president. Syria seeks closer ties with Egypt.

United Arab Republic

1958 February - Syria and Egypt join the United Arab Republic (UAR). Egyptian president Gamal Abdel Nasser heads the new state. He orders the dissolution of Syrian political parties, to the dismay of the Baath party, which had campaigned for union.

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Israel occupied the Golan Heights in 1967 during the Six Day War

• Golan Heights profile

1961 September - Discontent with Egyptian domination of the UAR prompts a group of Syrian army officers to seize power in Damascus and dissolve the union.

1963 March - Army officers seize power. A Baathist cabinet is appointed and Amin al-Hafez becomes president.

Rise of Assad

1966 February - Salah Jadid leads an internal coup against the civilian Baath leadership, overthrowing Amin al-Hafez and arresting Salah al-Din al-Bitar and Michel Aflaq. Hafez al-Assad becomes defence minister.

1967 June - Israeli forces seize the Golan Heights from Syria and destroy much of Syria's air force in the Six Day War with Egypt, Jordan and Syria.

1970 November - Hafez al-Assad overthrows president Nur al-Din al-Atasi and imprisons Salah Jadid.

1971 March - Assad is elected president for a seven-year term in a plebiscite.

1973 - Rioting breaks out after Assad drops the constitutional requirement that the president must be a Muslim. He is accused of heading an atheist regime. The riots are suppressed by the army.

War with Israel

1973 October - Syria and Egypt go to war with Israel but fail to retake the Golan Heights seized during the 1967 Arab-Israeli war.

1974 May - Syria and Israel sign a disengagement agreement.

1975 February - Assad says he's prepared to make peace with Israel in return for an Israeli withdrawal from "all occupied Arab land".

1976 June - Syrian army intervenes in the Lebanese civil war to ensure that the status quo is maintained, and the Maronites remain in power.

1978 - In response to the Camp David peace agreement between Egypt and Israel, Assad sets out to gain strategic parity with Israel.

Riots

1980 - After the Islamic Revolution in Iran, Muslim groups instigate uprisings and riots in Aleppo, Homs and Hama. Assad begins to stress Syria's adherence to Islam. Syria profile

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Hafez al-Assad brought stability, but did so through repression

• Obituary: Syria's shrewd master

Syria profile

1980 - Muslim Brotherhood member tries to assassinate Assad.

1980 September - Start of Iran-Iraq war. Syria backs Iran, in keeping with the traditional rivalry between Baathist leaderships in Iraq and Syria.

1981 December - Israel annexes the Golan Heights.

Uprising in Hama

1982 February - Muslim Brotherhood uprising in the city of Hama. The revolt is suppressed by the military, whom rights organisations accuse of killing tens of thousands of civilians.

1982 June - Israel invades Lebanon and attacks the Syrian army, forcing it to withdraw from several areas. Israel attacks the PLO base in Beirut.

1983 May - Lebanon and Israel announce the end of hostilities. Syrian forces remain in Lebanon.

1983 - Assad suffers a heart attack, according to reports denied by authorities. Assad's brother Rifaat apparently prepares to take power.

1984 Rifaat is promoted to the post of vice-president.

Return to Lebanon

1987 February - Assad sends troops into Lebanon for a second time to enforce a ceasefire in Beirut.

1990 - Iraq invades Kuwait; Syria joins the US-led coalition against Iraq. This leads to improved relations with Egypt and the US.

1991 October - Syria participates in the Middle East peace conference in Madrid and holds talks with Israel that founder over the Golan Heights issue.

1994 - Assad's son Basil, who was likely to succeed his father, is killed in a car accident.

Rifaat sacked

1998 - Assad's brother Rifaat is "relieved of his post" as vice-president.

Damascus urban sprawl

1999 December - Talks with Israel over the Golan Heights begin in the US, but are indefinitely postponed the following month.

Assad succession

2000 June - Assad dies and is succeeded by his second son, Bashar.

2000 November - The new President Assad orders the release of 600 political prisoners.

2001 April - Outlawed Muslim Brotherhood says it will resume political activity, 20 years after its leaders were forced to flee.

2001 5 May - Pope John Paul II pays historic visit.

2001 June - Syrian troops evacuate Beirut, redeploy in other parts of Lebanon, following pressure from Lebanese critics of Syria's presence.

2001 September - Detention of MPs and other pro-reform activists, crushing hopes of a break with the authoritarian past of Hafez al-Assad. Arrest continue, punctuated by occaisional amnesties, over the following decade.

2001 November - British PM Tony Blair visits to try shore up support for the campaign against terror. He and President Assad fail to agree on a definition of terrorism.

Tensions with US

2002 May - Senior US official includes Syria in a list of states that make-up an "axis of evil", first listed by President Bush in January. Undersecretary for State John Bolton says Damascus is acquiring weapons of mass destruction.

2003 April - US threatens sanctions if Damascus fails to take what Washington calls the "right decisions". Syria denies US allegations that it is developing chemical weapons and helping fugitive Iraqis.

2003 September - President Assad appoints Mohammed Naji al-Otari prime minister.

2003 October - Israeli air strike against Palestinian militant camp near Damascus. Syria says action is "military aggression".

2004 January - President Assad visits Turkey, the first Syrian leader to do so. The trip marks the end of decades of frosty relations, although ties sour again after the popular uprising in 2011.

2004 March - At least 25 killed in clashes between members of the Kurdish minority, police and Arabs in the north-east.

2004 May - US imposes economic sanctions on Syria over what it calls its support for terrorism and failure to stop militants entering Iraq.

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Syria and Lebanon

The killing of former Lebanese PM Rafik Hariri sparked anti-Syrian protets in Beirut

• Syria withdrawal: Lebanese speak
• Q&A: Syria and Lebanon

2004 September - UN Security Council resolution calls for all foreign forces to leave Lebanon.

Pressure over Lebanon

2005 February-April- Tensions with the US escalate after the killing of former Lebanese PM Hariri in Beirut. Washington cites Syrian influence in Lebanon. Damascus is urged to withdraw its forces from Lebanon, which it does by April.

2005 October - Interior minister and Syria's former head of intelligence in Lebanon, Ghazi Kanaan, dies in what officials say is suicide. UN inquiry into assassination of former Lebanese PM Rafik Hariri implicates senior Syrian officials.

2005 December - Exiled former vice-president Abdul Halim Khaddam alleges that Syrian leaders threatened former Lebanese PM Hariri before his assassination.

2006 February - Danish and Norwegian embassies in Damascus are set on fire during a demonstration against cartoons in a Danish newspaper portraying the Muslim Prophet Muhammad.

2006 September - Attack on the US embassy in Damascus. Four gunmen open fire and throw grenades but fail to detonate a car bomb. Three of them are killed, one is captured.

Diplomatic overtures

2006 November - Iraq and Syria restore diplomatic relations after nearly a quarter century.

2007 March - European Union relaunches dialogue with Syria.

2007 April - US House of Representatives Speaker Nancy Pelosi meets President Assad in Damascus. She is the highest-placed US politician to visit Syria in recent years. Secretary of State Condoleezza Rice meets Foreign Minister Walid Muallem the following month in the first contact at this level for two years.

2007 May - Leading dissident Kamal Labwani and prominent political writer Michel Kilo are sentenced to a long jail terms, only weeks after human rights lawyer Anwar al-Bunni is jailed.

Israeli strike

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Mystery site

Israeli forces destroyed what they said was a nuclear facility under construction. Syria says it was an unused military facility

2007 September - Israel carries out an aerial strike against a site in northern Syria that it said was a nuclear facility under construction. In 2011 the UN's IAEA nuclear watchdog decides to report Syria to the UN Security Council over its alleged covert nuclear programme reactor programme at the site.

2008 March - Syria hosts Arab League summit. Many pro-Western states send lower-level delegations in protest at Syria's stance on Lebanon.

2008 April - The US accuses North Korea of having helped Syria to build a secret nuclear reactor at the site bombed by Israel in 2007.

International acceptance

2008 July - President Assad meets French President Nicolas Sarkozy in Paris. The visit signals the end of the diplomatic isolation by the West that followed the assassination of former Lebanese PM Rafik Hariri in 2005. While in Paris, President Assad also meets the recently-elected Lebanese president, Michel Suleiman. The two men agree to work towards the establishing of full diplomatic relations between their countries.

2008 September - Damascus hosts four-way summit between Syria, France, Turkey and Qatar, in a bid to boost efforts towards Middle East peace. Explosion kills 17 on the outskirts of Damascus, the most deadly attack in Syria in several years. Government blames Islamist militants.

Diplomatic thaw continues

2008 October - Syria establishes diplomatic relations with Lebanon for first time since both countries established independence in 1940s.

Syria has a sizeable Christian community

2009 March - Jeffrey Feltman, acting assistant US secretary of state for the Near East, visits Damascus with White House national security aide Daniel Shapiro in first high-level US diplomatic mission for nearly four years. Meets Foreign Minister Walid Muallem.

Trading launches on Syria's stock exchange in a gesture towards liberalising the state-controlled economy.

2009 May - Syrian writer and pro-democracy campaigner Michel Kilo is released from prison after serving three-year sentence.

2009 June - The UN nuclear watchdog, the IAEA, says traces of undeclared man-made uranium have been found at second site in Syria - a reactor in Damascus. The IAEA was investigating US claims that the site destroyed in the 2007 Israeli raid was a nuclear reactor.

2009 July - US special envoy George Mitchell visits for talks with President Assad on Middle East peace.

2009 August - Iraq and Syria recall their envoys in a deepening rift over charges of responsibility for a string of deadly bomb attacks in Baghdad. They restore ties later in 2010.

2010 February - US posts first ambassador to Syria after a five-year break.

2010 May - US renews sanctions against Syria, saying that it supports terrorist groups, seeks weapons of mass destruction and has provided Lebanon's Hezbollah with Scud missiles in violation of UN resolutions.

Nationwide uprising

2011 March - Protests in Damascus and the southern city of Deraa demand the release of political prisoners. Security forces shoot a number of people dead in Deraa, triggering days of violent unrest that steadily spread nationwide over the following months.

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2011 protests

Pro-democracy protests erupted in 2011; the government responded with violence

• Syria: Origins of the uprising
• Bashar al-Assad's inner circle

The government announces some conciliatory measures in an attempt to damp down unrest. . President Assad releases dozens of political prisoners and dismisses the government, and in April lifts the 48-year-old state of emergency. However, he accuses protesters of being Israeli agents.

2011 May - Army tanks enter Deraa, Banyas, Homs and suburbs of Damascus in an effort to crush anti-regime protests. US and European Union tighten sanctions. President Assad announces amnesty for political prisoners.

2011 June - The government says that 120 members of the security forces have been killed by "armed gangs" in the northwestern town of Jisr al-Shughour. Troops besiege the town and more than 10,000 people flee to Turkey. President Assad pledges to start a "national dialogue" on reform.

2011 June - The IAEA nuclear watchdog decides to report Syria to the UN Security Council over its alleged covert nuclear programme reactor programme. The structure housing the alleged reactor was destroyed in an Israeli air raid in 2007.

Opposition organises

2011 July - President Assad sacks the governor of the northern province of Hama after mass demonstration there, eventually sending in troops to restore order at the cost of scores of lives. Opposition activists meet in Istanbul to form a unified opposition.

2011 October - Newly formed Syrian National Council says it has forged a common front of internal and exiled opposition activists. Russia and China veto UN resolution condemning Syria.

2011 November - Arab League votes to suspend Syria, accusing it of failing to implement an Arab peace plan, and imposes sanctions. Army defectors target a military base near Damascus in the Free Syrian Army's most high-profile attack since protests began. Government supporters attack foreign embassies.

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Civil war

The uprising against President Assad gradually turned into a full-scale civil war

• Syria's unwinnable war
• Covert help for rebels

2011 December - Syria agrees to an Arab League initiative allowing Arab observers into the country. Thousand of protesters gather in Homs to greet them, but the League suspends its mission in January because of worsening violence.

Twin suicide bombs outside security buildings in Damascus kill 44, the first in a series of large blasts in the the capital that continue into the following summer. Opposition accuses government of staging these and subsequent attacks.

UN pressure

2012 February - Russia and China block a UN Security Council draft resolution on Syria, and the government steps up the bombardment of Homs and other cities, recapturing the Homs district of Baba Amr the following month. The UN says that more than 7,500 people have died since the security crackdown began.

2012 March - UN Security Council endorses non-binding peace plan drafted by UN envoy Kofi Annan. China and Russia agree to support the plan after an earlier, tougher draft is modified. The UN statement falls short of a formal resolution, and violence continues into the summer.

2012 May - UN Security Council strongly condemns the government's use of heavy weaponry and the militia killing of more than a hundred civilians in Houla, near Homs. France, the UK, Germany, Italy, Spain, Canada and Australia expel senior Syrian diplomats in protest.

2012 June - President Assad tells his reshuffled government that they face "real war", indicating the authorities' conviction that the conflict will be long-lasting and require the sidelining of all other priorities.

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Opposition rifts

Divisions and concern about the role of Islamists have bedevilled the opposition

• Guide to the Syrian opposition

Turkey changes rules of engagement after Syria shoots down a Turkish plane that strayed into its territory, declaring that if Syrian troops approach Turkey's borders they will be seen as a military threat.

2012 July - Free Syria Army blows up three security chiefs in Damascus and seizes Aleppo in the north. A government offensive to recapture the city makes only limited headway.

2012 August - The government suffers further blows. A UN General Assembly resolution demands that President Assad resign, high-level defections gather pace - most notably Prime Minister Riad Hijab - and US President Obama warns that use of chemical weapons would tilt the US towards intervention.

UN appoints veteran Algerian diplomat Lakhdar Brahimi as new UN-Arab League envoy for Syria after resignation of Kofi Annan.

2012 September - The Free Syrian Army claims responsibility for two explosions at the military headquarters in Damascus. The government says four guards were killed in the "suicide attacks".

2012 October - Syria-Turkish tension rises when Syrian mortar fire on a Turkish border town kills five civilians. Turkey returns fire and intercepts a Syrian plane allegedly carrying arms from Russia. Both countries ban each other's planes from their air space.

Fire in Aleppo destroys much of the historic market as fighting and bomb attacks continue in various cities.

UN-brokered ceasefire during the Islamic holiday of Eid al-Adha breaks down as government continues attacks.

2012 November - Several major opposition forces unite as National Coalition for Syrian Revolutionary and Opposition Forces at meeting in Qatar, including the Syrian National Council. Arab League stops short of full recognition. Islamist militias in Aleppo, including the Al-Nusra and Al-Tawhid groups, refuse to join the Coalition, denouncing it as a "conspiracy".

Israeli military fire on Syrian artillery units after several months of occasional shelling from Syrian positions across the Golan Heights, the first such return of fire since the Yom Kippur War of 1973.

2012 December - The US joins Britain, France, Turkey and Gulf states in formally recognising Syria's opposition National Coalition as "the legitimate representative" of the Syrian people.

The conflict in Syria displaced millions of people, many of whom sought refuge in camps in Jordan, Turkey and Lebanon

2013 January - Syria accuses Israeli jets of attacking a military research centre near Damascus, but denies reports that lorries carrying weapons bound for Lebanon were hit. Unverified reports say Israel had targeted an Iranian commander charged with moving weapons of mass destruction to Lebanon.

International donors pledge more than $1.5bn (£950m) to help civilians affected by the conflict in Syria.

2013 March - Syrian warplanes bomb the northern city of Raqqa after rebels seize control. US and Britain pledge non-military aid to rebels, and Britain and France propose lifting European Union arms embargo. Rebel National Coalition elects US-educated technocrat Ghassan Hitto as interim "prime minister".

2013 April - US and Britain demand investigation into reports government forces used chemical weapons. Prime Minister Wael Nader Al-Halqi narrowly escapes death in bomb attack in centre of Damascus.

Opposition National Coalition chairman Moaz al-Khatib resigns, accusing foreign backers of trying to manipulate the group. His successor is veteran socialist George Sabra, leader of the older opposition Syrian National Council.

2013 May - Israeli and Syrian Army exchange fire in the Golan Heights.

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Chemical arms claims

Government forces have faced - and denied - repeated allegations of chemical weapons use

• Chemical weapons allegations
• Syria's chemical weapons stockpile

EU leaders agree not to renew the bloc's arms embargo on Syria, in a step seen as potentially freeing EU countries to arm the rebels.

2013 May-June - Government and allied Hezbollah forces recapture the strategically-important town of Qusair between Homs and the Lebanese border. Rebel commanders complain that arms supplies taper off over international concerns about Islamists in the opposition camp.

2013 July In a leadership overhaul, Saudi-backed Ahmed Jarba replaces interim figure George Sabra as leader of the main opposition National Coalition, defeating a Qatar-backed rival. Interim opposition PM Ghassan Hitto quits, citing his inability to form a government in rebel-held territory.

Rebels say they capture Khan al-Assal, the last major government-held town in the west of Aleppo Province, after two months of successful government offensives.

2013 August - Rebels and Western governments accuse pro-Assad forces of using chemical weapons in an attack that killed more than 300 people near Damascus. The Syrian government says it was the rebels that used chemical weapons. The US and Britain take contingency measures for possible military action. Russia and China warn against any attack on Syria.