Supporting and defending an ally is one thing. If Iran launches a war against Israel, we should and will help defend them.
But we cannot allow ourselves to be manipulated into another Middle-Eastern war simply because the government of Israel decides to initiate a pre-emptive strike. The very idea is absurd on its face.
You want our protection, you take it on our terms. Period.
Iran's warhawks are saber-rattlers. Their rhetoric is intended for internal political purposes. They know what would happen if they ever attacked Israel (which they could have already done with chemical weapons - but haven't). They don't want to be bombed into oblivion. They aren't insane.