This is utterly false.  Google '2013 Syria Gas Attack' and read, for example, BBC's 9/24/13 summary.  Every credible analysis, including the United Nations (summarized in an address by Sec Gen Ban Ki-moon), Human Rights Watch and President Obama's administration, conclude beyond doubt that Assad's regime alone was responsible for the sarin attack that left about 1,300 dead.  Deal with facts, please, and not misinformation.