The problems between the United States and Iran are based on geopolitics and not on ideology. In the end Iran only poses a limited geopolitical threat to U.S. interests. On the side, Russia is a serious threat to global American interests and even American soil, just think of the Russian nuclear arsenal. Essentially the problems between the U.S. and Russia vastly outweight the problems between U.S. and Iran. An alliance is always formed out of fear from a 3rd party, that fear which Iran and the U.S. share is a re-emerging Russia.