Action: Syria uses chemical weapons on its own people.
Possibility #1
Trump Reaction: Trump responds with a proportional missile attack against a Syrian airbase and says the United States won't tolerate another world state using chemical weapons.
Liberal/Leftist/Progressive/Democrat Response: They get their collective panties in a bunch because Syria didn't attack the United States so they claim Trump had no right to become the aggressor with them. Clearly Trump is a warmongering hothead and he isn't fit to be President.
Possibility #2:
Trump Reaction: Trump simply verbally condemns the attack while ultimately doing nothing (you know... just like what our previous President would have done).
Liberal/Leftist/Progressive/Democrat Response: They become enraged that Syria can violate international law and engage in genocidal activities against its own citizens and Trump sits idly by and does nothing about it. It's just further proof that he hates Muslims, women, and children and is unfit to be President.
In other words, Trump could develop a cure for cancer and make it free to everyone and leftists would still manage to find something to bitch about. "But, but, but... why didn't he develop it sooner? What took him so long? If he wasn't spending so much time on Twitter he could have discovered and made it available quicker than he did. Think of all the needless suffering and deaths of innocent women and children, many of whom were undoubtedly Muslim, that resulted because he was too busy being a pompous prick! Clearly he doesn't care about people and he's unfit to be President!"