As a U.S. and Russian-backed cease-fire is to go into effect, here are some of the key events since the uprising against President Bashar Assad began:
— March 2011: Protests erupt in the city of Daraa over security forces' detention of a group of boys accused of painting anti-government graffiti on the walls of their school. On March 15, a protest is held in Damascus' Old City. On March 18, security forces open fire on a protest in Daraa, killing four people in what activists regard as the first deaths of the uprising. Demonstrations spread, as does the crackdown by Assad's forces.
— April 2011: Security forces raid a sit-in in Syria's third-largest city, Homs, where thousands of people tried to create the mood of Cairo's Tahrir Square, the epicenter of protests against Egypt's autocrat Hosni Mubarak.
— June 2011: Police and soldiers in Jisr al-Shughour in northeastern Syria join 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: 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.
— August 2012: Kofi Annan quits as U.N.-Arab League envoy after his attempts to broker a cease-fire failed. Obama says the use of chemical weapons in Syria would be a "red line" that would change his thinking about military action.
— March 2013: After advancing in the north, rebel forces capture Raqqa, a city of 500,000 people on the Euphrates River and the first major population center controlled by the opposition.
— May-June 2013: Backed by thousands of Lebanese Hezbollah fighters, Assad's forces re-capture the strategic town of Qusair from rebels, near the border with Lebanon.
— 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.
— 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 spreads, pitting a variety of Islamic groups and moderate factions against the al-Qaida-breakaway Islamic State of Iraq and the Levant.
— February 2014: Two rounds of peace talks led by U.N.-Arab League mediator Lakhdar Brahimi in Geneva end without a breakthrough.
— May 9: Rebels withdraw from the old quarter of the central city of Homs in a significant symbolic victory for the government.
— May 13: Brahimi resigns as U.N.-Arab League envoy to Syria, marking a second failure by the United Nations and Arab League to end the civil war.
— June 3: Syrians in government areas vote in presidential elections. Assad, one of three candidates, overwhelmingly wins with 88.7 percent.
— June: The Islamic State group, as the Islamic State of Iraq and the Levant is now known, seizes large parts of northern and western Iraq. In control of around a third of Syria and Iraq, it declares a self-styled Islamic caliphate.
— July 3: Islamic State group takes control of Syria's largest oil field, al-Omar, after fierce battles with the Nusra Front, al-Qaida's branch in Syria.
— Aug. 19: Islamic State militants release video of the beheading of American journalist James Foley, the first of five Westerners to be beheaded by the IS group.
— Mid-September: IS begins offensive to take Syrian Kurdish town of Kobani, on the Turkish border.
— Sept. 23: U.S.-led coalition begins airstrikes against Islamic State group targets in Syria.
— January 2015: U.N. estimates Syria's conflict has killed at least 220,000 people and uprooted nearly a third of the prewar population of 23 million from their homes.
— Jan. 26: With the help of U.S.-led airstrikes, Kurdish fighters take control of Kobani.
— Feb. 3: IS releases a video of captured Jordanian pilot Muath al-Kaseasbeh being burned to death in a cage.
__ March 28: The northwestern city of Idlib falls to Islamist groups led by the Nusra Front.
__ May 6: President Bashar Assad acknowledges serious setbacks for his military.
__ Sept. 30: Russia begins launching airstrikes in Syria in support of Assad's forces.
__ Dec. 18: The U.N. Security Council adopts resolution 2254 endorsing a road map for a transitional period in Syria that includes parliamentary and president elections as well as a new constitution within 18 months.
__ Feb. 3: Indirect peace talks between the Syrian government and opposition in Geneva collapse few days after starting, over a Russian-backed Syrian army offensive in Aleppo.
__ Feb. 22: The U.S. and Russia announced a partial cease-fire in Syria will start on Feb. 27.