Even if we could go to war on legitimate reasons, what could we achieve?
removing ISIL? then we will just go in, kill some bastards and probably leave since we can't afford to stay there between all that mess for a long time. if we go in, we must go in hard, cut off their arm, not shave their beard (though many of them could use a shave with only a gilette mach 3...) and try to influence the situation in our way for our short/long-term interests.
hell, if our national interests/safety were at a big stake i would even approve a false flag (but not killings, just rockets on empty ground, outdated vehicles etc) and go for our national interests, because only honesty and soft power is not gonna make us become a bigger player if we dont learn assert ourselves with hard power when we can.