The US/Israel and allies do not have to invade Iran. All they have to do is weaken them, and send the nuclear program back a decade or two.
I do agree however that, if they do invade, it would weaken the US strategical footprint globally, and some of our enemies might seize the opportunity to make a point. Namely Russia in some of her former republics, and NK. China might make a move on Taiwan, knowing that the US engaged in iran would be more willing to make a deal for Taiwan. There are lots of potential scenarios..
The allies would need to avoid boots on the ground in order to maintain.