Paperback published '74-'78 Russia & China conquer US with poison gas?- 200 years later American finds superweapon saucer in secret cave

[JDNewman]

I read in library '79-'81 kind of a Kamandi vibe. Hero uses the superweapon to fight Russian Chinese. Shades of the "Yang" on Star Trek episode "The Omega Glory". In the opening an American army soldier waits behind machine gun guarding the entrance to cave -200 years later hero finds entrance and thinks the man is still alive because he was preserved by gas? Saucer or stealth weapon works by cybernetic headset? Thanks for any help!