So I stand by my first statement on the thread.
The deterrent should be to not be a bellend. If you have decided to push the button and start a nuclear war then, frankly, I don't think you give two ****s if one is coming back at you. You've already lost it.
You fire a nuclear missile, one is coming back.
If you're mental enough to fire the first one, I can't see you receiving one back and then going "Do you know what? That's fair. Let's call it quits".