Chemical weapons are believed to have been first used in mid 2012. On the 2th of August US president Obama proclaimed that the use of chemical weapons is a red line for the US.
The most violent chemical weapons attack was on opposition-held Douma to the east of Damascus on the 23th of August 2013
The UN Security Council Resolution 2118 on the 27th of September 2013 states that the Assad regime needs to follow a timeline for the neutralization of its chemical weapons stockpiles and chemical weapons production facilities.
On the 4th of April 2017 the Assad regime executed a chemical weapons attack on Khan Sheykhoun in Idlib. Regime airplanes bombed the area of Khan Sheykhoun in southern Idlib with chlorine gas killing more than 100 civilians, most of whom were children, and affecting 500 more.
After the attack on Khan Sheykhoun, the use of chemical weapons has been seen in 11 more different places.
The latest chemical weapons attack was on the 7th of April 2018 again in Douma where 40 civilians lost their lives.