Read about a few things I was not aware of in this article.
http://theweek.com/speedreads/691527/officials-conclude-russia-knew-syria-chemical-attack-advance
The U.S. claimed Iraq had WMDs. More recently, the U.S. claimed Syria used gas on its own people in 2013–a claim that was debunked by the U.N. Meanwhile, there is actually an independent international organization that investigates these kinds of things.
Who are you going to believe?