Trump's failure (if one wants to call it that) is not that he may bomb Syria, it's that he made the promise that he would if it happened again.
Trump may be between a rock and a hard place. It depends on whether he saw Obama’s first line to be now US policy. If he saw it that way then he felt the need to act when the Obama line was crossed under his own presidency, then that may be why he felt and continues to feel that we must act. I think Trump drew his own line in the sand since Obama’s first line, which puts him on the same hook Obama got on a few years ago.
In my perfect world, Trump should have ignored the Obama line that Obama stupidly drew. I never agreed with Obama having drawn it in the first place and I'm sure, with the exception of Mcain and Graham, most republicans disagreed with the Obama line as well. Their problem with Obama’s line was that he lost us credibility for failing to act when it was crossed.
Somehow, this all got forgotten.
You don’t go to war just because some jackass drew a “line in the sand,” do you?