Should US forces leave Iraq to the Islamists at certain civil war would follow and the radicals would prevail. Once in power they would unite with radical Iran and effectively rule the entire middle east.
With unlimited petro dollars and access to advanced weapons from Pakistan, Russia and China a united Islam would seriously threaten Europe, India and Africa. Maybe even Russia. No doubt Islam foot hold in the Balkans would be reinforced, Serbia, Greece and Italy would be targeted for conquest. The Islamic "stain" states on Russia's southern border would be embolden and Chechian would become Russia's Iraq.
Anyone that can not see the horrible consequences for the west, that a US defeat in Iraq would bring, is either blinded by hate of the GOP or an out and out traitor to the USA and Western civilization.
US/UK defeat
Can't forget the Brits.