On Aug. 21, 2013, a Damascus suburb called Ghouta was attacked with sarin gas, resulting in the deaths of hundreds of civilians.

President Barack Obama had warned that the United States would take military action if President Bashar al-Assad of Syria used chemical weapons. The attack on Ghouta crossed Mr. Obama’s “red line,” but he chose coercive diplomacy instead of military action.

Syria acknowledged that it had chemical weapons. The United States and Russia reached a deal in mid-September 2013 under which Syria had to destroy its chemical weapons program.

The Syrian government hurriedly acceded to the Chemical Weapons Convention and turned over what it said was its complete chemical weapon arsenal to a team of experts from the United Nations and the Organization for the Prohibition of Chemical Weapons, which monitors adherence to the convention.