Trust me they will trump up a rationale for it...
Do you feel there was an imminent threat when Reagan invaded Grenada?
Obama will not be setting any precedent if he bombs syria and cites the war powers act... Reagan did the same thing to Libya.
The reality is that this law has been used to justify all sorts of questionable actions... Like it or not, Obama would not be doing anything previous presidents of both parties have done if he does act without congressional approval.