The US Federal Reserve has printed money so that foreign countries can trade with it. If all those countries dump the US dollar in favor of the Euro, the US dollar will be devalued enormously. The cost of oil will go through the roof, and that could devastate the US economy. Probably would, in fact. High energy costs have an inflationary effect, and that would prompt banks to raise interest rates. Businesses would have more difficulty securing loans, and with higher costs their share prices would drop making it harder to raise capital. Generally the US economy would suffer. Also, when inflation is high in the US, south asian, east asian, european, and african stock markets also drop because western banks start to sell off foreign equities. Basically the whole world economy would go down the shitter.

I think this was one of the main reasons why the US went to war in Iraq, because they were the first OPEC country to switch to Euros. Not sure why the UK backed them. Maybe they bought the story about WMD, or maybe they don't like the Euro being used to trade oil either, or maybe it was just about the balance of power. Iraq has now been forced to trade oil in US dollars again, regardless of deleterious effects on the Iraqi economy. Now that military action has proven to be a strategy of questionable benefit, the US probably won't try it again unless there is another terrorist attack on US soil. In that case they will probably go after Iran, whether or not they were involved, just for strategic purposes.

The US will probably prefer the diplomatic route with Iran, using the nuclear issue as a backdrop for negotiations on energy trade. Meanwhile Iran is strengthening ties with Russia, China, Pakistan, India, Germany, and France. The US, while still a member of the security council, is the "odd one out". They want sanctions, but the other members of the security council don't, since they might benefit from the Iranian Oil Bourse. Already China and Russia are increasing their holdings of Euros. It might not happen suddenly. It might happen slowly, which might be a good thing, who knows. But the issue has already been suppressed for years by the media, drowned by news about terrorists, WMD, and wars.

Personally, IMO it would be good if the US weren't so dependent on OPEC oil, e.g. via massive marketing success of Clever Cars. If they weren't though, they wouldn't have so much influence. Nor interest I suppose.