The issue, as outlined in the wikipedia entry, is that they aren't using Federal Reserve Notes to transact their oil internationally.
Of course they don't.
We don't trade with them. If the US cares so much about what currency they use, why did we place an embargo on them?
You're saying that it makes sense for the US to invade their country, leading to US deaths, negative public opinion, and a debilitated Iranian economy just so that they use USD instead of the Euro? That is not sound logic.
I think it would make more sense for the US to just lift the embargo. Why go to war?
It's irrelevant whether the United States trades with Iran; it's a currency issue. Exporting Federal Reserve notes maintains the value of the US dollar, and the United States uses military force to ensure that the US dollar stays as the world reserve currency through the Petrodollar.