[Kale]

That and this would give them a chance to finally pop off some nukes. I'm guessing the aliens just used a suppression field to prevent detonation or simply death ray'd the nukes out of the sky. A lot of world powers would be tempted to at least make a go of it before rolling over in surrender. That's probably why the aliens had to smash the Russians so thoroughly: Discourage anyone else from trying it.