In 2013, a brutal chemical weapons attack targeted civilians in Douma Syria.

A U.S. Inteligence report had warned the Whitehouse that Al Nusra Front were planning a Sarin attack, they also knew it wasn't Syrian chemicals used in the attack itself.

But Obama blamed Assad anyway @irishmaninrussia

Here's a few articles:

https://truthout.org/video/seymour-hersh-obama-cherry-picked-intelligence-on-syrian-chemical-attack-to-justify-us-strike/

https://www.bellingcat.com/news/mena/2017/06/25/will-get-fooled-seymour-hersh-welt-khan-sheikhoun-chemical-attack/

https://www.lrb.co.uk/the-paper/v35/n24/seymour-m.-hersh/whose-sarin