I had read once that prior to the US lead invasion of Afghanistan in 2001 the Iranians were looking at the possibility of invading Afghanistan and taking out the Taliban. A group which was a side of Islam they wanted to avoid spreading.
The Iranians never went far with that plan but lets say, in a grand overture of peace and diplomacy, Bush negotiated a deal with the Iranians which would get them to to invade with the promise of aid, the glory of a military victory over an enemy of Islam, and the hope of gaining territory. The idea of an invasion, spearheaded by US airpower clearing away a path for an invasion of Northern alliance and Iranian forces is interesting.
Would the Iranians have gone for it? Would they even have been capable of the dealing with the logistics of such an invasion? What would the consequences have been?
BTW, yes I know we were not getting along with the Iranians. However their is an old saying "the enemy of my enemy is my friend".