https://www.usatoday.com/story/news/world/2017/04/06/us-launches-strikes-against-syria-heres-what-we-know/100148690/

Back in 2013, President Obama set a “red line” against the use of chemical weapons by Syrian President Bashar Assad. The regime proceeded to use the weapons to kill 1,400 civilians, but Obama did not attack — a move Trump and other Republicans widely criticized as making America look weak.

In the wake of that episode, Assad agreed to turn over his stockpile of chemical weapons. This week’s chemical attack clearly violated that pledge. Trump called the attack — which killed at least 86 people, 27 of them children —  “a disgrace to humanity” and “truly one of the egregious crimes.”