No one does anything for a single reason. Part of the reason for going to war is always to get practice, as gnu said, and to use up old equipment and buy new and make money off that, and get promotions and medals, and so on. But when we talk about the reasons for a particular war, we assume all that and we just talk about why they chose this particular country to run their racket in this time.

That may involve multiple reasons also. The invasion of Iran won't be done solely to punish them for setting up a non-dollar oil bourse and trying to undermine the world financial system. There's also the fact that Iran has massive oil and gas reserves, and it occupies the south shore of the Caspian Sea with its massive reserves and so it would be convenient as a pipeline route to the ocean and to Pakistan. And they're conspicuously non-aligned, so it's a great opportunity to expand US power in general and to show other nations what happens if you don't submit to US hegemony. And of course we have to stop them from enriching uranium for the medical research reactor we gave them during the Shah's regime with the understanding that they would be eternally grateful and submissive and would always buy the uranium from us. And maybe the racket isn't paying off as well as they'd hoped in Afghanistan so they want to try Iran instead.