Don't get me started on Iran, but I have to pull you up on that point ... most external fighters in Iraq are from Saudi Arabia, with the majority of arms from Saudi and Syria.
It doesn't matter where the people come from, they are useless without weapons and funding, which is provided by Iran. Iran can only benefit from a weak Iraq, a weak neighbor is easier to conquer, they have their eyes set on Saudi Arabian and Israel. The wealth oil from Saudi Arabia will fund their coalition against Israel.
They want israel out, that's for sure. Do they want to conquer iraq? I don't think so, they're just hoping for a shia majority rule in that country, hoping it will one day be a fundamentalist islamic state. that's there dream.