In order of risk—

Scenario One:
Probably the same thing that happened the last time. Trump will give Putin a phone call and tell him that he plans a strike at such-and-such a place, in 48 hours, and the Russians have time to pull their people and gear out, plus whatever Syrians and Iranians they feel are worth saving. The Americans get to feel self-righteous, and the other side gets to play the indignant victim without really losing too much of value. Nothing much of consequence actually happens. This is the most likely scenario.

Scenario Two:
But not necessarily. Maybe this time, Trump is really pissed and wants to teach the bastards a lesson. His advisers, fearful of possible escalation, will probably try to talk him out of it, but the President may actually decide to take the initiative. So he orders a cruise missile or air strike against some Syrian base or command center–or maybe several–selected to really hurt the Syrian war effort, or at least, cost them a lot in men and materiel. The bad guys will know this is coming because they won’t get the phone call that goes with Scenario One. They will be ready for us.

Scenario Three:
Trump decides to go to the real root of the problem; he’ll target Russian or Iranian depots, radars, air bases, port facilities, whatever it takes to cause the body bags to start going back to Tehran and Moscow. At the very least, it will cause them to spend a lot of money to repair the damage and beef up security and air defenses in case Trump tries it again. Russia’s military is first-rate, but its small (Russia has about the same GDP as Canada or Italy). Its questionable how long they can continue to replace their losses in a real war of attrition. The US, OTOH, has limitless resources, but they are already spread too thin, all over the world. If things escalate, and they start going bad for either side, things could easily get out of hand.
Somebody could really screw up, or get really lucky, and it will be a whole new ball game afterwards. And when the going gets tough, the tough get weird.

Its possible, especially in the case of Scenarios Two and Three, the effectiveness of these options will depend on how much force the US and its allies can assemble on relatively short notice, and on how effective an air defense the defenders can put together. It will also depend on how much and how long the strikes last, and how many losses we will be willing to accept (yes, there will be losses). I don’t know what assets we have in the area, and how quickly they can be supplied and reinforced, or what the other side is capable of doing. But I’m sure everybody involved knows pretty well what everybody elses capabilities and limitations are. Everyone will know ahead of time what the butcher’s bill is going to be. Or at any rate, what it should be if things go as expected.

What we don’t know is what happens if the situation gets out of control And war has a way of surprising everybody involved.

This is a lot worse than Korea, or even the S China Sea.