Americans have such a short memory. The neocon fear mongering, "If we leave the Middle East it will become a terrorist base and they'll follow us home," is just a variation of the communist "domino theory" that obviously was proven false. Just as back then, when there was a "communist behind every bush", now there's a "terrorist behind every bush."
There's also an implication that the United States is unable to defend itself -- that unless we get them "over there" they'll show up on our streets and there's nothing we can do to stop them. So we have to "get them over there" or we're all gonna die!
If we secured our borders and focused our intelligence efforts in the right places, we'd be much safer than we are now. And the incentive for young Muslims to join radical groups to "fight the invaders" would wither away.