Here's a look at some of the key events in the Syrian uprising as the conflict marks its third anniversary:
— March 2011: Protests erupt in Daraa, Syria, over security forces' detention of a group of boys accused of painting anti-government graffiti on the walls of their school. On March 18, security forces open fire on a protest in the southern city, killing four people in what activists regard as the first deaths of the uprising. Demonstrations spread, as does the crackdown by President Bashar Assad's forces.
— June 2011: Police and soldiers in Jisr al-Shughour in northeastern Syria join forces with the protesters they were ordered to shoot, and the uprising claims control of a town for the first time. Elite government troops, tanks and helicopters retake the town within days.
— August 2011: U.S. President Barack Obama calls on Assad to resign and orders Syrian government assets frozen.
— July 2012: A bombing at the Syrian national security building in Damascus during a high-level government crisis meeting kills four top officials, including Assad's brother-in-law and the defense minister.
— Summer 2012: Fighting spreads to Aleppo, Syria's largest city and its former commercial capital. Over time, rebels seize control of about half of the city, but the battle there rages to this day, leaving much of Aleppo in ruins.
— August 2012: Obama says the use of chemical weapons in Syria would be a "red line" that would change his thinking about military action.
— November 2012: The Syrian National Coalition is created, bringing together the main opposition factions. The umbrella group is hampered from the outset by crippling infighting and accusations that its members are out-of-touch exiles.
— March 2013: After advancing in the north, rebel forces capture Raqqa, a city of 500,000 people on the Euphrates and the first major population center controlled by the opposition. That month, the number of United Nations-registered refugees tops 1 million, half of them children.
— May-June 2013: Regaining the offensive with the help of thousands of Hezbollah fighters, Assad's forces re-capture the strategic town of Qusair near Lebanon's border.
— June 2013: U.S. officials conclude that Assad's forces had used chemical weapons against the opposition. Obama authorizes direct support for the rebels.
— August-September 2013: A chemical weapons attack in the Damascus suburbs kills hundreds. Obama, blaming Assad's government, says the U.S. has a responsibility to respond and puts it up to a vote in Congress. Russia proposes instead that Syria give up its chemical weapons, averting military strikes.
— September 2013: Eleven rebel groups leave the Syrian National Coalition and form their own alliance intended to create an Islamic state.
— October 2013: Syria destroys its chemical weapons production equipment. The number of Syrian refugees registered with the U.N. tops 2 million.
— January 2014: Infighting among rebels, pitting a variety of Islamic groups and moderate factions against the al-Qaida-breakaway Islamic State of Iraq and the Levant, kills nearly 700 people in nine days. The first batch of toxic chemicals also is shipped out of Syria.
— February 2014: Peace talks led by U.N.-Arab League mediator Lakhdar Brahimi in Geneva end without progress. Brahimi says: "I am very, very sorry, and I apologize to the Syrian people that their hopes which were very, very high that something will happen here."
— March 15, 2014: The Syrian conflict marks its third anniversary.