The thing is, much like MAD (which IMO was responsible for there being no nuclear war back when), they'd just never know for sure. Any other way of 'scorched earth' would allow them to rebuild--and we know how they love doing that. So, nukes. In a fight we'd be decimated very quickly. Our military could buy us enough time to detonate. Let 'em know that if they insist on playing, we're gonna pitch.