We have several time-bomb treaties with Israel.
An interesting one is that if we "loan" money to Israel, the US will make the payments if Israel doesn't. That's why they have never defaulted - they can't.
Another is that if Israel is attacked the US will come to her aid. All Israel has to do is provoke an attack or make it seem as if she has been attacked and the US is obliged to go to war.
The US may not have a problem with Iran's army, but the aftermath would be bloody. After all, Iraq is now a dictatorship without much love for the US and we'd have to stage some military things from there. Or try Afghanistan, with it's impossible logistics.