Seymour Hersh, one of the few American journalists still worth his salt - and who's now apparently only able to be published in Germany, since his findings contradict too much with the "official" narrative - just did a good write-up on the deliberations that took place within the Trump White House in the days following Assad's alleged chemical attack in early April.

Basically, Hersh argues that the entire intelligence community knew it wasn't a sarin attack done by Assad or the Russians, but Trump could not be persuaded by experts, choosing instead to believe the videos of the White Helmets:

https://www.welt.de/politik/ausland/article165905578/Trump-s-Red-Line.html