Perspective

Less than 100 days into the Trump Administration.  Right off the top, there was the Yemen raid - followed of course by the it's-Obama's-fault standard defense to the problems and deaths raid.  Then I learned that in the short duration of the Trump Administration, his efforts resulted in more civilian deaths than since 2013 until the end of the Obama Administration. 
THEN Trump authorized the bombing in Syria. 
All this from the candidate who campaigned on America First.  It is possible that he was simply listening to recommendations of the generals in the administration who might be the only adults in the room and, as has been widely reported, he listens to the recommendations of the last person in the room.
Until he talked about babies dying in chemical attack.  Since when does he care about 89 lives, including some babies, in Syria?  He doesn't care about the seniors who won't eat if he defunds Meals on Wheels, if EPA head Pruitt reverses the ban on a farm pesticide the causes brain damage in infants.  It is public knowledge that Trump urged Obama to avoid military action in Syria when 1400 people were killed by chemical warfare. 
Trump has not staffed his Department of State and his Secretary of State has completely reversed his position toward Syria over a couple days.  All Trump has advising him are some very competent generals and two feuding advisors - one who is his 36 year old son-in-law and the other is an Alt-right anti-Semitic nationalist.  We can be pretty sure that Trump isn't making decisions on the basis of historical perspective and discreet, well thought policy.
What changed?  Trump is under suspicion for collusion in the Russian hacking.  Russia is a strong, public ally of the Syrian leader, Assad.  What Trump rally needs is evidence that he is not in Russia's pocket.
Either prospect is terrifying - either he is seeing his military potential as a Presidential toy or as a means to prove he doesn't have ties to Russia or to divert attention from the investigation into the Russian hacking and possible Trump collusion.