Re: Er, what?
Ever heard of ISIS and Al Qaeda? I'm sure they've been in the news the odd time or two...
As it turns out, the times we've allowed them to hold territory, they've recruited and trained large numbers of people for terrorist attacks back in Europe. Hence making the calculation that allowing them to hold territory might be a worse idea than the alternative of using military force to stop them.
You may think that calculation is wrong, but you can't call it irrational.