Timeline of the Syrian civil war and America's response
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 United Nations blaming three attacks on the Syrian government and a fourth on the Islamic State terror 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 American attack on the Syrian government.
Mr Trump blamed Syrian president Bashar Assad for the attack and called on the international community to join him in trying to end the bloodshed.
Here is a timeline of events in Syria leading up to Tuesday's attack.
:: 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 Bashar Assad's forces.
:: April 2011: Security forces raid a sit-in in Syria's third-largest city, Homs, where thousands tried to create the mood of Cairo's Tahrir Square, the epicentre of protests against Egypt's autocrat Hosni Mubarak.
:: August 18 2011: US president Barack Obama calls on Assad to resign and orders Syrian government assets frozen.
:: Summer 2012: Fighting spreads to Aleppo, Syria's largest city and its former commercial capital.
:: August 20 2012: Mr Obama says the use of chemical weapons would be a "red line" that would change his calculus on intervening in the civil war and have "enormous consequences".
:: March 19 2013: The Syrian government and opposition trade accusations over a gas attack that killed 26 people, including more than a dozen government soldiers, in the town of Khan al-Assal in northern Syria. A UN investigation later finds that sarin nerve gas was used, but does not identify a culprit.
:: 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.
- US-led coalition killed more Syrian civilians than Isis or Russia in March, figures show
- Mosul: US-led air strikes 'leave 230 civilians dead', reports local media
- US strike on Afghanistan's Kunduz hospital was deliberate, says Médicins Sans Frontiéres boss Christopher Stokes
- Claims Jabhat al-Nusra aided by Turkish intelligence in sarin gas attacks
- Syria rebels 'may have used chemical weapons'
:: 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 in the legislature.
:: 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.
:: October 14 2013: Syria becomes a signatory to the Chemical Weapons Convention, prohibiting it from producing, stockpiling or using 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 were not fully accounted for and that it retained supplies.
:: September 23 2014: The U.S. launches air strikes on IS targets in Syria.
:: August 7 2015: The UN Security Council authorises the 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.
:: August 24 2016: The joint OPCW-UN panel determines the Syrian government twice used helicopters to deploy chlorine gas against 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 IS used mustard gas.
:: 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.
:: 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.
:: April 4 2017: US president Donald Trump issues a statement saying that the "heinous" actions of Assad's government are the direct result of Obama administration's "weakness and irresolution".
:: April 5 2017: Trump says Assad's government has "crossed a lot of lines" with the suspected chemical attack in Syria.
:: April 6 2017: The US fired a barrage of cruise missiles into Syria in retaliation for the 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.