For some time , though Syria denied possession of chemical weapons , Syria was believed to have the world 's third-largest stockpile of chemical weapons , after the United States and Russia . With the outbreak of the Syrian Civil War in 2011 concerns were raised about both the security of Syria 's chemical weapon sites and about the potential use of chemical weapons . On 21 August 2013 , rockets containing the chemical agent sarin struck several opposition-controlled or disputed areas of the Ghouta suburbs of Damascus , resulting in the death of more than 300 people and injury to thousands in the Ghouta chemical attack . President Barack Obama used the phrase " red line " in reference to the use by Syria of chemical weapons . The United States and other Western countries blamed the Syrian government for the attack , while Syria blamed civil war opposition forces . In response to Ghouta , a coalition of countries led by the United States and France , which support the rebels , threatened air strikes on Syria . Russia , a key ally of Syria , along with China had earlier blocked efforts by the United States , France , and the UK to secure United Nations Security Council approval for military intervention . The United States and France headed a coalition of countries on the verge of carrying out air strikes on Syria
Ghouta chemical attack
Ghouta chemical attack
Destruction of Syria's chemical weapons