Wow, okay, a thread on an "Over Kill" weapon of sorts.
What about something which leaves the country intact once it has been activated? Hard to concieve of, perhaps.
Surely one that doesn't expend any ammunition at all? For me it would have to be one which removes the will to fight in the enemy completely. Psychological warfare of some kind. The room for abuse would be massive though, like any weapon.
But...if all else fails, send in the Mechs/Armoured Cores/Gundams etc.
Its certainly more fun that way.