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The US attack on a Syrian air base came after years of heated debate and deliberation in Washington over intervention in the bloody civil war.
Chemical weapons have killed hundreds of people since the start of the conflict, with the UN blaming three attacks on the Syrian government and a fourth on the Islamic State group. One of the worst yet came on Tuesday in rebel-held northern Idlib and killed dozens, including women and children.
That attack prompted US President Donald Trump, on day 77 of his presidency, to dramatically shift US policy, with the first direct US attack on the Syrian government.
Mr Trump blamed Syrian President Bashar al-Assad for the attack and called on the international community to join the US in trying to end the bloodshed.
Here is a timeline of key events in the Syrian civil war.
First deaths in uprising against government
March 11, 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 al-Assad's forces.
Security forces work to halt Egypt-style sit-in
April, 2011
Security forces raid a sit-in in Syria's third-largest city, Homs, where thousands of people are trying to recreate the mood of Cairo's Tahrir Square, the epicentre of protests against Egypt's autocrat Hosni Mubarak. By the end of the year, Syria has been suspended by the Arab League and Mr Assad's troops are engaged in a war with rebel forces, led by deserters gathered under the loose banner of the Free Syrian Army.
Obama freezes Syrian government assets
August 18, 2011
Then-US president Barack Obama calls on Mr Assad to resign and orders Syrian government assets in the US to be frozen.
Fighting in Syria spreads to Aleppo
Summer, 2012
Fighting spreads to Aleppo, Syria's largest city and its former commercial capital.
Obama draws 'red line' against chemical weapons
August 20, 2012
Mr Obama says the use of chemical weapons in Syria would be a "red line" that would change his calculus on intervening in the civil war and have "enormous consequences".
Civilians killed in sarin attack
August 21, 2013
Hundreds of people suffocate in rebel-held suburbs of the Syrian capital, with many suffering from convulsions, pinpoint pupils, and foaming at the mouth. UN investigators visit the sites and determine that ground-to-ground missiles loaded with sarin were fired on civilian areas while residents slept. The US and others blame the Syrian government, the only party to the conflict known to have sarin gas.
Obama seeks authorisation for punitive strikes
August 31, 2013
Mr Obama says he will go to Congress for authorisation to carry out punitive strikes against the Syrian government, but appears to lack the necessary support on Capitol Hill.
UN orders destruction of chemical weapons
September 27, 2013
The UN Security Council orders Syria to account for and destroy its chemical weapons stockpile, following a surprise agreement between Washington and Moscow, averting US strikes. The Security Council threatens to authorise the use of force in the event of non-compliance.
Syria signs Chemical Weapons Convention
October 14, 2013
Syria becomes a signatory to the Chemical Weapons Convention, prohibiting it from producing, stockpiling or using chemical weapons.
Doubts over Syria's claim it has removed chemical weapons
June 23, 2014
The Organisation for the Prohibition of Chemical Weapons says it has removed the last of the Syrian government's chemical weapons. Syrian opposition officials maintain that the government's stocks are not fully accounted for, and that it retains supplies.
US launches airstrikes on IS targets in Syria
September 23, 2014
The US and allied nations launch airstrikes on Islamic State group targets in Syria.
UN orders probe amid reports of chlorine gas attacks
August 7, 2015
The UN Security Council authorises the Organisation for the Prohibition of Chemical Weapons (OPCW) and UN investigators to probe reports of chemical weapons use in Syria, as reports circulate of repeated chlorine gas attacks by government forces against civilians in opposition-held areas. Chlorine gas, though not as toxic as nerve agents, can be classified as a chemical weapon depending on its use.
UN finds Syria used helicopters for chlorine gas attacks
August 24, 2016
The joint OPCW-UN panel determines the Syrian government twice used helicopters to drop chlorine gas on its opponents in civilian areas in the northern Idlib province. A later report holds the government responsible for a third attack. The attacks occurred in 2014 and 2015. The panel also finds that the Islamic State group used mustard gas.
Russia, China veto sanctions against Syria
February 28, 2017
Russia, a stalwart ally of the Syrian government, and China veto a UN Security Council resolution authorising sanctions against the Syrian government for chemical weapons use.
About 60 killed in suspected nerve gas attack
April 4, 2017
At least 58 people are killed in what doctors say could be a nerve gas attack on the town of Khan Sheikhoun in the rebel-held Idlib province. Victims show signs of suffocation, convulsions, foaming at the mouth and pupil constriction. Witnesses say the attack was carried out by either Russian or Syrian Sukhoi jets. Moscow and Damascus deny responsibility.
Trump blames Obama administration for Assad's actions
April 4, 2017
US President Donald Trump issues a statement saying that the "heinous" actions of Mr Assad's government are the direct result of Obama administration's "weakness and irresolution."
Trump steps up rhetoric against Syria
April 5, 2017
Mr Trump says the Assad government has "crossed a lot of lines" with the suspected chemical attack in Syria.
US fires missiles at Syrian targets in response to chemical weapons attack
April 7, 2017
The US fires a barrage of cruise missiles into Syria in retaliation for this week's gruesome chemical weapons attack against civilians. It is the first direct American assault on the Syrian government and Mr Trump's most dramatic military order since becoming President. Mr Trump says the strike on Syria is in the "vital national security interest" of the United States.
Topics: unrest-conflict-and-war, terrorism, syrian-arab-republic
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