I'm not fully up to date on this situation but from my understanding it can be broken down into a few points.
1. Trump, with no real proof, decided Assad gassed his own people.
2. Trump, despite it having nothing to do with him, decided that meant he should drop 59 missiles on a Syrian base - proof be damned.
3. He did this without consulting Russia, his best friends, whilst in the presence of his nemesis from China - despite the presence of Russian soldiers on the base.
4. Plus, all this happened outside of congressional approval as per usual.
So we're three months in and not only is Syria going to become even less stable, but US-Russia relations are going to shit again despite Trump apparently planning on fixing that situation. Three months in and already a diplomatic incident with the potential to start a world war. Because Trump has pissed off the people he was apparently going to make nice with. Now can we say we told you so?
Onward to a few of the things people have said/will say more of.
Firstly, if this does lead to war - a very real possibility - it won't just be the US and Russia. If those two countries go to war, everyone does. For starters, any war the US is involved in Australia, New Zealand, the UK and probably Canada also get yanked into because of the treaties we have in place. Secondly, Russia also have allies. It'd spiral into a very shitty and potentially nuclear situation.
Secondly, the fact that Clinton and Obama would likely have done something is irrelevant. They didn't make this decision, Trump did. You can't try and distract from how very foolish this was on Trump's part by trying to point fingers at other politicians. I am so very tired of people trying to do that. As aside though, they also wouldn't have done anything in such a breathtakingly stupid way.