"Trump’s abrupt [act] is incredibly risky. With Russian + Iranian troops in Syria, escalating U.S. attacks could trigger a wider war."

1.   Ok, so why wasn't the Russian intervention equally risky and provocative?  

2.   Why would Russia want to fight a large war?  The US and its allies have ten times the resources that Russia has.

3.    Doing nothing can also be provocative.  Read "The Collapse of the Third Republic".  France was so scared it could not find the will to get a German brigade out of the  Saarland when France had crushing military superiority; couldn't find the will to intervene in Spain ....  Couldn't intervene over the Anschluss. ... Lost Czecho at Munich.   Each failure weakened France's position.

Clausewitz noted that aggressors don't want war.  They are glad to accept submission.  The general who is truly successful is the one who forces the enemy to surrender without fighting.

It is tricky to know when "enough is enough", but there must be a line somewhere.