Bashar al-Assad's forces have used chemical weapons many times before - according to Organisation for the Prohibition of Chemical Weapons (OPCW) and United Nations investigations. Human Rights Watch has documented 85 chemical weapons attacks in Syria since 2013. People reported incidents of chemical weapons use specifically in Douma in January 2018; Russia vetoed a potential United Nations mission to investigate. The Arms Control Association reported two smaller chlorine gas attacks in Douma on 7 March and 11 March.
Douma had been under rebel control since 18 October 2012, and, with the rest of the Eastern Ghouta region, under siege since April 2013. The Rif Dimashq offensive (February–April 2018), code-named Operation Damascus Steel, a military offensive launched by the Syrian Arab Army (SAA) and its allies on 18 February 2018 to capture the rebel-held territory. The Jaysh al-Islam rebel coalition controlled Douma. By mid-March, rebel territory in Eastern Ghouta had reduced to three pockets, one in the south around Hamouria held by Faylaq al-Rahman; a second in the west around Harasta held by Ahrar al-Sham; as well as Douma in the north held by Jaysh al-Islam. In the second half of March, the other two pockets were secured via evacuation deals between the rebels, Syria, and Russia. On 31 March, the last of the evacuations was conducted and the Syrian army declared victory in Eastern Ghouta, while the rebels that were still holding out in Douma were given an ultimatum to surrender by the end of the day.