A cease-fire that for two months brought relief to war-stricken Syrians has effectively collapsed, particularly in northern Syria, which has seen intense fighting and a military buildup. Here are some of the key events in the conflict:
— 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 President Bashar 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 to be 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.
— 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, 2014: Rebels withdraw from the old quarter of the central city of Homs in a significant symbolic victory for the government.
— May 13, 2014: 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, 2014: Assad re-elected to a seven-year term in elections held in government areas and condemned by the opposition.
— June 2014: 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, 2014: 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, 2014: 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 2014: IS begins offensive to take Syrian Kurdish town of Kobani, on the Turkish border.
— Sept. 23, 2014: 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, 2015: With the help of U.S.-led airstrikes, Kurdish fighters take control of Kobani.
— Feb. 3, 2015: IS releases a video of captured Jordanian pilot Muath al-Kaseasbeh being burned to death in a cage.
__ March 28, 2015: The northwestern city of Idlib falls to Islamist groups led by the Nusra Front.
__ May 6, 2015: President Bashar Assad acknowledges serious setbacks for his military.
__ Sept. 30, 2015: Russia begins launching airstrikes in Syria in support of Assad's forces.
__ Nov. 14, 2015: Seventeen nations meeting in Vienna adopt a timeline for a transition plan in Syria that includes a new constitution as well as U.N.-administered parliamentary and presidential elections within 18 months.
__ Dec. 18, 2015: The U.N. Security Council adopts resolution 2254 endorsing the Vienna road map.
__ Feb. 3: Indirect peace talks between the Syrian government and opposition in Geneva collapse a few days after starting, over a Russian-backed Syrian army offensive in Aleppo.
__ Feb. 27: A U.S. and Russia engineered partial cease-fire comes into effect in Syria.
__March 14: A new round of indirect peace talks resumes in Geneva and ends 10 days later with no breakthrough.
__April 13: Syria holds parliamentary elections in government-held parts of the country.
__ April 27: Another round of indirect peace talks in Geneva ends with no breakthrough. Fighting and a military buildup in the north spell the end of the cease-fire, although none of the parties formally renounce it.
__April 28: A wave of airstrikes and shelling on the northern city of Aleppo kills more than 60 people, including at least 27 killed in an airstrike on an MSF-supported hospital and nearby buildings