Every U.S. soldier is taught the importance of complying with law, including international law, in every task he or she undertakes. They are also taught that dishonor or worse follows from violating this law. Many of the soldiers and all of the lawyers involved in the Syria planning will surely feel at least a little uneasy about a military action that the President acknowledges does not pass the test of international legality.
That's the real bitter irony. Compromising the law to an effective end is one thing; but then to be unable to help ordinary Syrians and then to damage the USA in so many ways (both aspects, simultaneously) makes it all the sadder.