The only time that American troops ever directly supported Israel was in the Persian Gulf War when American Patriot missiles were deployed to defend Israel from Saddam's Scuds because it was felt that if Israel itself acted against them it might erode the already shaky alliance between Arab nations and America if Israel got involved. It was A-okay if a bunch of Arabs joined America in ousting the hated Saddam from Kuwait but another thing entirely to fight alongside Israel against another Arab nation.
America won't start a war with Iran or aid Israel in a war against Iran, at least not until Afghanistan and Iraq are secured which could take another 10 years. Because right now American troops are spread too thin as it is.
Assuming that Ahmedinajad is wanting to have nuclear weapons as a final step from nuclear energy, his actions are not that crazy at all. In fact, I'd argue I would be doing the same thing were I in his shoes.
If we put ourselves in Iran's position, wherever they look, they see American forces. They are surrounded by them. And history tells that where there are American troops there is war - if there isn't one going on, there will be.
NATO in Afghanistan, coalition in Iraq, in Kuwait, in Oman, UAE, Azerbaijan, Uzbekistan, Turkey...
And knowing that Bush Administration sat there in the White house and UN and lied to their people and the world to go to war with Iraq over nothing, it is no wonder that such makes Iran nervous.
If America goes to war with Iran, there won't be anyone else going in with it except Israel and it will spell the end of America.
I doubt the Chinese and the Russians would be too upset...