The was on ISIS wasn't waged exclusively or even mainly by the West. It was waged on the ground primarily by the local armed forces - Iraqi army in Iraq, YPG militias in both Iraq and Syria etc - who were actively resisting the attempts to take over their communities by force and in some cases outright genocide them. The West provided active military support including air strikes and occasional on-the-ground assistance, but you can't reduce the reasons for that war to "a few westerners beheaded"; you have to look at the totality of crimes against humanity perpetrated by ISIS on its occupied territories to get a comprehensive casus belli on behalf of the opposing forces primarily responsible for all the destruction.