The Taliban, ISIS, and Al Qaeda are fundamentalist Sunni Muslims and account for about 15 percent of the world's Muslims. Shiite Muslims are dominant in Iran and are also the largest sect in Iraq. Those two sects don’t tend to get along very well and I think that’s a sort of Middle Eastern cold war that has been a part of that region for awhile. Iraq though was more on their bad side from an earlier war they fought against each other (that the US supported Saddam Hussein during).

Potentially Iran knew about 9/11 beforehand, but to me that’s highly doubtful. Hell, GW Bush would’ve looked for any shred of evidence to go after Iran and had something like that existed, we would’ve likely already gone to war with them. As someone else noted, the Saudis may have known something about it beforehand, but it may have just been intelligence agents and not the actual royal family, I can’t recall what any of the investigations turned up on that.

That was one of the “issues” American foreign policy had for a number of years. We treated Iran like enemy number one, then weakened two surrounding countries that they weren’t on good terms with (Iraq/Afghanistan), allowing Iran to extend their influence into both countries and now everybody in the region is basically united against us given our insane policy towards Israel.