Russians must be kicking themselves now, that they didn't do this when they should have (the late 1960s). This picture explains why:
(The characters on the map say "China" in Chinese.)
There's also this other source (the book "A Great Wall: Six Presidents and China"):
I thought this was well known.
This wasn't the first time China almost got nuked. MacArthur wanted to nuke China during the Korean War.