The Trump administration's latest threat to punish Syria if it uses chemical weapons follows various gas attacks and hundreds of deaths over the course of the Arab country's six-year civil war.
Some events related to chemical weapons use in Syria over time:
Aug. 20, 2012: President Barack Obama says using chemical weapons is 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 U.N. investigation later finds that sarin nerve gas was used, but does not identify a culprit.
Aug. 21, 2013: Hundreds of people suffocate to death in rebel-held suburbs of Amman, the Syrian capital, with many convulsing, displaying pinpoint pupils and foaming at the mouth. U.N. investigators visit the sites and determine that ground-to-ground missiles loaded with sarin were fired on civilian areas while residents slept. The U.S. and others blame the Syrian government, the only warring party at the time known to have sarin gas.
Aug. 31, 2013: Obama says he will go to Congress for authorization to carry out punitive strikes against the Syrian government, but appears to lack support among U.S. lawmakers.
Sept. 27, 2013: The U.N. Security Council orders Syria to account for and destroy its chemical weapons stockpile, following a surprise agreement between Washington and Moscow that averts U.S. strikes. The Security Council threatens to authorize the use of force in the event of noncompliance.
Oct. 14, 2013: Syria becomes a signatory to the Chemical Weapons Convention, prohibiting it from producing, stockpiling or using chemical weapons.
June 23, 2014: The Organization 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.
Aug. 7, 2015: The U.N. Security Council authorizes the OPCW and U.N. investigators to examine possible 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.
Aug. 24, 2016: The joint OPCW-U.N. 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 the Islamic State group used mustard gas.
Feb. 28, 2017: Russia, a staunch ally of the Syrian government, and China veto a U.N. Security Council resolution authorizing sanctions against the Syrian government for chemical weapons use.
April 4, 2017: Almost 90 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. The U.S. and its allies blame Syria for the attack. Assad's government and its allies deny the accusation.
April 7, 2017: President Donald Trump orders 59 Tomahawk cruise missiles fired at the Shayrat air base in Syria in response to the chemical weapons attack. Syria and its military supporters, Russia and Iran, denounce the U.S.
June 26, 2017: The White House issues a statement late in the evening saying the U.S. has "identified potential preparations for another chemical weapons attack by the Assad regime that would likely result in the mass murder of civilians, including innocent children." The statement says the activity is similar to what it noticed before the April 4 attack and warns that if "Mr. Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price."