Trump’s attack on Syria dealt with several issues that should help his popularity:
I creates a contrast with Obama’s publicly drawing a red line on the use of chemical weapons in Syria and then doing nothing about it. Trump looks strong and decisive in comparison, and it pleases the liberal establishment.
It helps to overcome Trump’s perceived softness on Putin. He attacked Putin’s ally, Assad, and even put Russian troops in peril. For the moment at least, he and Putin are on opposite sides in Syria.
It demonstrates to North Korea that Trump is not afraid to use force, and thus constitutes an implicit threat to North Korea.
Liberals in general like the attack on Syria because it inhibits the use of nuclear weapons and attacks on civilians and children. Thus, the attack tends to roll back their perception of him as a far right ideologue.
Liberal talking heads have expressed concern that Trump has no strategy to bring about regime change or end the war in Syria, but Trump can let this attack stand alone if he wants. He can describe it as a response to an inhumane violation of international law, not the beginning of regime change.