The reality is that if the US had decided to act alone, before others were ready, then this would have removed all the pressure from other countries and their participation would have evaporated. The US should not have to fix all world problems, and we need the rest of the world to get used to that. Europeans will have to act, especially in their own backyard, if they regard the threat as grave enough.
BTW, this conflict is far from over. There are still all kinds of problems, including the potential destruction of huge amounts of oil production infrastructure, which could make the price of gas go through the roof again. I hope that Obama prosecutes price-fixers this time. Bush certainly wouldn't/didn't because he was in bed with them as an oil tycoon himself.