A timeline of some key events in the Syrian uprising:
March 15, 2011 — Activists call for a "Day of Rage" across Syria, inspired by other popular uprisings across the Arab world. In February, several youths were arrested in the southern town of Daraa for writing graffiti calling for the downfall of the regime of President Bashar Assad.
March 18, 2011 — Activists say five people were killed as security forces dispersed crowds in the southern town of Daraa — one of several demonstrations across the country — in the first deadly violence reported in the uprising. Over the next days, Daraa was reported sealed off, with no one allowed to enter.
March 23, 2011 — Protests continue in Daraa, and Syrian state media release images of guns, hand grenades, bullets and stacks of Syrian currency said to be seized from a mosque in the town.
March 25, 2011 — Troops open fire on protesters in several cities and crowds clash on the streets of the capital of Damascus.
April 26, 2011 — Thousands of soldiers backed by tanks and snipers open fire on civilians in Daraa and two other locations, according to witnesses. Armed security agents conduct house-to-house sweeps. Neighborhoods are sectioned off and checkpoints are erected. Electricity, water and cellphone services are cut. At least 11 people are killed and 14 others lay in the streets, either dead or gravely wounded.
May 18, 2011 — The U.S. imposes sanctions on Assad and six senior Syrian officials for human rights abuses, freezing any assets they have in U.S. jurisdiction. The Swiss government passes a measure restricting arms sales to Syria and freezing the assets and banning the travel to Switzerland of 13 senior Syrian officials.
June 7, 2011 — Details emerge of a mutiny by Syrian soldiers in the northern town of Jisr al-Shughour, where 120 troops were killed, according to the government. The loss of control appears to expose cracks in the autocratic regime and its ability to squelch ongoing protests.
Aug. 5, 2011 — After several days of a ferocious assault on the city of Hama, the epicenter of anti-regime protests, hundreds are left dead by Syrian security forces backed by tanks and snipers. Corpses are scattered in the streets and Hama hospitals are overwhelmed with casualties. President Barack Obama calls the reports "horrifying."
Aug. 18, 2011 — The United States, Britain, France and Germany and the European Union demand that Assad resign, saying he is unfit to lead.
Oct. 4, 2011 — Russia and China veto a European-backed U.N. Security Council resolution that threatened sanctions against Syria if it didn't immediately halt its military crackdown against civilians.
Oct. 6, 2011 — As Assad attends a ceremony to mark the 38th anniversary of the 1973 Arab-Israeli war, Syrian troops storm villages close to the border with Turkey, hunting armed military defectors, according to activists.
Oct. 24, 2011 — The U.S. pulls its ambassador out of Syria over security concerns.
Nov. 8, 2011 — The U.N. human rights office says the death toll for the uprising is 3,500.
Nov. 12, 2011 — The Arab League votes to suspend Syria's membership, a stinging rebuke to a regime that prides itself as a bastion of Arab nationalism.
Nov. 27, 2011 — The Arab League overwhelmingly approves sanctions against Syria to pressure Damascus to end its crackdown, an unprecedented move by the League against an Arab state.
Dec. 12, 2011 — U.N. rights chief Navi Pillay says more than 5,000 have died in the Syrian conflict.
Dec. 23, 2011 — Two car bombs explode near intelligence agency compounds in Damascus, killing 44 people in the first suicide attacks since the uprising began.
Dec. 28, 2011 — Syrian security forces open fire on thousands of anti-government protesters in the central city of Hama, killing at least six people — one day ahead of a visit by Arab League observers on a mission to end the crackdown. The government also releases 755 prisoners following a report by Human Rights Watch accusing authorities of hiding hundreds of detainees from the observers.
Jan. 2, 2012 — An explosion hits a gas pipeline in central Syria and the government blames "terrorists." The opposition accuses the government of playing on fears of religious extremism and terrorism to rally support behind Assad.
Jan. 6, 2912 — Exactly two weeks after twin bombings in Damascus, another explosion rips through a busy intersection and police bus in the capital, killing at least 25 people.
Jan. 11, 2012 — Gilles Jacquier, a French cameraman, is killed in Homs — the first Western journalist to die in Syria's uprising.
Jan. 28, 2012 — The Arab League halts its observer mission in Syria because of escalating violence that killed nearly 100 people the past three days, as pro-Assad forces battled dissident soldiers in a belt of suburbs on the eastern edge of Damascus in the most intense fighting yet so close to the capital.
Jan. 30, 2012 — A gas pipeline near the Lebanese border is blown up, according to state-run media. Activists report gunfire and explosions in the Damascus suburbs. A day later, troops crush pockets of resistance in the suburbs, with the death toll at about 100.
Feb. 3, 2012 — Activists say an assault by government forces in Homs kills more than 200 people and wounds hundreds.
Feb. 4, 2012 — Russia and China veto a resolution in the U.N. Security Council that backed an Arab League plan that calls for Assad to step down. The double-veto outrages the U.S. and European council members who feared it would embolden the Assad regime.
Feb. 6, 2012 — The Obama administration closes the U.S. Embassy in Damascus and pulls all American diplomats out of Syria.
Feb. 22, 2012 — French photojournalist Remi Ochlik and American-born journalist Marie Colvin of Britain's Sunday Times are killed by government shelling of Baba Amr, a key rebel-held neighborhood in Homs. Two other journalists are wounded.
Feb. 26, 2012 — Syria holds a referendum on a new constitution, a gesture by Assad to placate the opposition. The West dismisses the vote as a "sham." The next day, activists say the death toll from nearly a year of unrest has surpassed 8,000, with most of the victims civilians.
March 1, 2012 — Syrian troops take control of Baba Amr after a government assault that raged for weeks. The rebels retreat, having run low on weapons under unbearable humanitarian conditions. Syria's main opposition group, the Syrian National Council, forms a military council to organize and unify all armed resistance.
March 8, 2012 — Syria's deputy oil minister announces his defection in an online video, making him the highest ranking official to abandon Assad's regime since the uprising began.
March 13, 2012 — Syrian military forces reportedly take control of the northern rebel stronghold of Idlib along the border with Turkey, a major base that army defectors had held for months. An international rights group said the army is mining the border with Turkey.
March 15, 2012 — On the first anniversary of the start of the uprising, thousands march in a pro-Assad rally in Damascus. Tanks and snipers continue to besiege Daraa. The U.N. secretary-general says more than 8,000 have been killed in the crackdown.