the united states will not invade iran, the purpose of iraq contrary to typical belief was not oil (although that was definitley beneficial) it is to pursue the so called pax american, similar to pax roma during the roman empire, the americans have since the end of the cold war lost there power in the eurasian region. This is and will prove to be in the future the most important region in the world. During the clinton administration he received a letter demanding that he attack iraq, lol of course this was before any even made up suspicion, they signatares on this letter were cheney and rumsfeldt, the actual arcitechts of the iraq war. These are individuals who not only benefited greatly from the pax america during the cold war but helped tremendously in the effectiveness of it (see cheney and his former think tank). If the us were to pick a nation to attack who would they go after. Well if they were going after north korea that could be potentially suicide, the terrain is similar to vietnam, very mountainous not to mention there is a large population and you have a psychopathic dictator with a nuke, well that would kinda ruin your day if he let that baby go. Second choice Iran, well iran to is a mountainous area, plus it is a country of roughly twice the population of iraq who strongly support there government, plus we know they still have wmd's so american forces would no doubt have to face these in iran. The last choice is Iraq, well iraq is flat. Not to mention it is a small country (30 million people) who for the most part do not support their leader. Plus we know they don't have wmd.
IMO if the united states were to undergo any new military action it would not be until iraq has restores its "freedom" and "sovereignty". I would expect somewhere in the south china sea to be the next area "liberated". The south China sea is the 3rd largest expected oil reserve in the world, after the persian gulf and the caspian sea and it is probably one of the least exploited. Also the united states would most certainly need some more support in the east eurasian area. Globalization has made this area on of the fastest growing regions in the world. This area has become crucial to the american economy and in all reality it would be a bad idea on their part not to gain some control in that region.