Why Are We Fighting ISIS?

Why, exactly, are we fighting ISIS?  If it is "to stop them over there before they come over here", then we could stop fighting them there with the understanding they don't attack Americans.  It would be pretty much over.  I mean their religion would call off the fatwa on us and bingo, bango, boom, done.
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ISIS is fighting our regional antagonists; Iran, Syria, Hezbollah and Iraq (which joined the group of nations that hates us right after we disposed Saddam Hussein).  So why are we helping them?  True, we like the Kurds, but I bet we could come to an understanding with them.
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Our allies - Saudi Arabia beheaded 47 "terrorists" this week.

I ask because of the recent news out of Saudi Arabia.  This New Year's they beheaded 47 people - including a cleric (from the minority group) for "terrorism" - a loose term in Saudi Arabia that includes saying something bad about the leaders of the country (link to Stars and Stripes).
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Last year they executed a couple of bloggers that were under 18 for terrorism.  In fact - about a year ago the Saudi Government defined terrorism as:

The law codified that the kingdom could prosecute as a terrorist anyone who demands reform, exposes corruption or otherwise engages in dissent or violence against the government.

That's a pretty broad definition of terrorism.
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So if we can support this country (Saudi Arabia) - why do we give a shit about ISIS?  Just wondering.