Boots on the ground is what gave ISIS traction in the first place.
That's a terrible way to characterize the issue, though. American military action obviously allowed ISIS to exist (through the destabilization of Iraq and political excommunication of essentially every Ba'athist), but what allowed ISIS to become the state it is today was the
withdrawal of American troops before the Iraqi government was at all capable to handle its internal threats.
The Iraq War was such a horribly selfish action because of the clear disregard for the well-being of the Iraqi government and its people. You can't invade a country and destroy a government without being able to see its political transition all the way through.
I thought rape is forbidden in Islam?
Mueller and Abu Bakr were legally married, in accordance to what passes for law in the Islamic State. Pagan slave women (who are not Jewish, Christian, or Muslim) are allowed to be kept as non-marital concubines, and groups like ISIS or Al-Shabaab see this as a scriptural endorsement of gang-rape.
But a lot of ISIS members aren't very familiar with the scripture anyway, and probably rape out of lust and hatred.