1969 Sino-Soviet War

This claim has appeared in a few other places, but apparently without the official sanction:

Liu Chenshan, the author of a series of articles that chronicle the five times China has faced a nuclear threat since 1949, wrote that the most serious threat came in 1969 at the height of a bitter border dispute between Moscow and Beijing that left more than one thousand people dead on both sides.

He said Soviet diplomats warned Washington of Moscow’s plans “to wipe out the Chinese threat and get rid of this modern adventurer,” with a nuclear strike, asking the US to remain neutral.

But, he says, Washington told Moscow the United States would not stand idly by but launch its own nuclear attack against the Soviet Union if it attacked China, loosing nuclear missiles at 130 Soviet cities. The threat worked, he added, and made Moscow think twice, while forcing the two countries to regulate their border dispute at the negotiating table.

Some observations:

1. Even if the USSR mooted the idea of a nuclear attack on China to the United States, it doesn’t mean that such an attack would actually have been carried out. Soviet conventional capabilities greatly exceeded Chinese, although perhaps not to the degree that the Russians could have ensured the destruction of China’s (fairly primitive) atomic forces without resort to nuclear attack. Suggesting to Washington that an attack was imminent may just have been an attempt to feel out the Nixon administration’s attitude towards China.
2. Even if the US insisted it would respond to an attack on China by nuking the USSR, it doesn’t mean that such a response would have been undertaken. Nixon would have every incentive to bluff in this situation, and I have serious doubts as to whether he would have been willing to go full Armageddon in defense of the PRC. Obviously, the US had made neither an explicit nor implicit security guarantee to Beijing, minimizing potential reputational effects of a non-response. Of course, the threat of a nuclear response against Moscow would carry its own costs without follow-through.
3. Launching nukes against China would have been an incomparably bad idea on the part of Moscow. Even assuming that the Russians managed to destroy the Chinese nuclear deterrent, it’s unlikely that the Russians would have been willing to completely destroy China as a political and cultural entity. We now know that the Soviet leadership was a) sensitive to international opinion, and b) at least somewhat nervous about the idea of butchering hundreds of millions of people. A nuclear attack to settle a border dispute would not have sat well with anyone in either the West or the Third World, and probably would have incurred serious resistance from within the CPSU.