I presume one place such proof might be is in the intelligence reports which convinced the President, his Defense Secretary, his cabinet, and his advisors that Assad really did launch a chemical gas attack against his people.
And evidently, the President made a judgement that something needed to be done about it. I, for one, am highly skeptical that President Trump would undertake such an action if he wasn't 100% convinced of Assad's guilt.
The armchair general second-guessers around here are really overreacting to this. I suspect ulterior motives in some cases.
One isolated cruise missile salvo and it's "Apocalypse Now" in the minds of the hand-wringers. The lack of trust and "fair weather friend" reaction is just astounding...
Being ISIS’s air force is what is “astounding”. They are having a nice celebration today. You should get yourself a candy.