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 Daraa — one of several demonstrations across the country — in the first deadly violence reported in the uprising. Unrest spreads in coming months.
—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 — U.S. imposes sanctions on Assad and senior Syrian officials for human rights abuses.
—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 stop ongoing protests.
—Aug. 5, 2011 — After days of 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. 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 threatens sanctions against Syria if it doesn't immediately halt its military crackdown against civilians.
—Oct. 24, 2011 — U.S. pulls its ambassador out of Syria over security concerns.
—Nov. 8, 2011 — U.N. human rights office puts death toll for the uprising at 3,500.
—Nov. 12, 2011 — Arab League votes to suspend Syria's membership, a stinging rebuke to regime that sees itself as bastion of Arab nationalism.
—Nov. 27, 2011 — Arab League overwhelmingly approves sanctions against Syria to pressure Damascus to end crackdown, an unprecedented move 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 — Back-to-back car bombs near Syria's intelligence agencies in Damascus kill at least 44 in first major attack in the heart of the capital. Syria's state-run TV blames al-Qaida militants.
—Dec. 28, 2011 — Syrian security forces open fire on thousands of anti-government protesters in the central city of Hama, one day ahead of a visit by Arab League observers on a mission to end the crackdown. The government 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. 28, 2012 — Arab League halts its observer mission in Syria because of escalating violence as pro-Assad forces battle dissident soldiers in Damascus suburbs.
—Feb. 3, 2012 — Activists say 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 backs an Arab League plan calling for Assad to step down. The double-veto outrages the U.S. and European council members who fear it will embolden the Assad regime.
—Feb. 6, 2012 — Obama administration closes U.S. Embassy in Damascus and pulls out all American diplomats.
—Feb. 26, 2012 — Syria holds referendum on a new constitution, a gesture by Assad to placate the opposition. The West dismisses the vote as a sham.
—March 1, 2012 — Syrian troops take control of shattered Baba Amr after a government assault that raged for weeks. The 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, 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.
—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.
—April 11, 2012 — Syria promises to comply with a U.N.-brokered cease-fire but carves out an important condition — that the regime still has a right to defend itself against terrorists that it says are behind the uprising. The agreement ultimately fails to hold.
—June 16, 2012 — U.N. observers suspend patrols in Syria due to escalating violence.
—July 18, 2012 — A blast at the National Security building in Damascus kills the defense minister and his deputy, who is also Assad's brother-in-law, and wounds the interior minister. Rebels claim responsibility.
—July 23, 2012 — Syria threatens to unleash chemical and biological weapons if the country faces a foreign attack, the country's first acknowledgement that it possesses weapons of mass destruction.
—Aug. 2, 2012 — Kofi Annan announces his resignation as U.N.-Arab League envoy to Syria after failing to broker a cease-fire despite developing a peace plan that the Syrian government initially agreed to but then failed to implement.
—Aug. 15, 2012 —U.N. Human Rights Council releases a report accusing Assad's forces and pro-government militiamen of war crimes during a May bloodbath in the village of Houla that killed more than 100 civilians, nearly half of them children. It says rebels were also responsible for war crimes in at least three other killings.
—Aug. 20, 2012 — Obama says U.S. will reconsider its opposition to military involvement in Syria if Assad's regime deploys or uses chemical or biological weapons, calling such action a "red line" for the United States.
—Nov. 11, 2012 — Syrian anti-government groups strike a deal to form a new opposition leadership that will include representatives from the country's disparate factions fighting to topple Assad's regime, responding to repeated calls from their Western and Arab supporters to create a cohesive and representative leadership.
—Dec. 3, 2012 — Speaking of chemical weapons, Obama says Assad should know "if you make the tragic mistake of using these weapons, there will be consequences and you will be held accountable."
—Jan. 6, 2013 — A defiant Assad blames "murderous criminals" for violence in Syria, ignores international demands to step down and pledges to continue the battle "as long as there is one terrorist left" in Syria.
—Feb. 12, 2013 — U.N. human rights chief says Syria's intensifying civil war has probably killed nearly 70,000 people, and blames U.N. Security Council for its failure to end the killings.
—April 25, 2013 — White House says U.S. intelligence indicates Assad has twice used chemical weapons in his country's civil war, but says the information isn't solid enough to warrant quick U.S. involvement in the 2-year-old conflict.
—May 27, 2013 — European Union ends its embargo on sending weapons to help Syrian rebels.
—June 13, 2013 — Obama authorizes sending weapons to Syrian rebels after White House discloses that U.S. has conclusive evidence Assad's government used chemical weapons on a small scale against opposition forces. U.N. human rights office raises overall death toll in the civil war to nearly 93,000 through the end of April 2013.
—Aug. 21, 2013 — Assad regime is accused of using chemical weapons in the Damascus suburbs to kill large numbers of civilians, including many children as they slept. The government denies using chemical weapons.
—Aug. 30, 2013 — Obama administration says it has "high confidence" that Syria's government carried out the chemical weapons attack that killed more than 1,400 people outside Damascus.
—Aug. 31, 2013 — Obama says he has decided the United States should take military action against Syria in response to the chemical weapons attack. But the president says he will seek congressional authorization for the use of force.