[NicknamedBob]

Ancient Mars, back when it had oceans, still had a lot of arable land area. Two nuclear devices would have been insufficient to do much damage to a global civilization.

Losing its atmosphere over thousands of years, on the other hand, would have been breathtakingly catastrophic.

[NoCmpromiz]

But if it were aliens perpetrating those thermonukes, why that would certainly be sufficient to bring about the end of Mars as they knew it... ;-)

Those aliens can do anything.