One of the closest brushes with nuclear war was Russia vs China
As they're now America's two top rivals, it's easy to forget that China and Russia aren't allies and actually have decades of regional rivalry and have been at each other's throats more than once. In fact, in 1970, the Soviet Union started asking around about whether or not anyone would really care if they launched a nuclear war against China.
Ya know, for world security and all that.
Russia and China try to smooth over their regional troubles in the common interest of trying to constrain America, even when Russia was the Soviet Union and the year was 1950. Russia and China sent pilots to North Korea to help fight American air power, downing and killing U.S. pilots. It was a real high-point for Soviet-Sino Relations.
But at the time, China was basically to the Soviet Union what North Korea is to China today. The Soviet Union was much larger and stronger, and it was embroiled in a battle of superpowers with the U.S. China was welcome on the playground as long as it was playing by the rules and backing up Soviet interests. But China wanted to become a nuclear power just like its big brother.
And so, in 1964, China detonated its first device, becoming the fifth country to become a nuclear power.
This combined with already simmering tensions over border conflicts and brought the two countries' relations to a low boil. Their troops fought skirmishes against one another on their shared border while both sides greatly built up their troops and their stockpiles of less-than-nuclear weapons like biological and chemical threats.
In 1969, this grew into the Sino-Soviet border conflict, a seven-month undeclared war between the two sides from March to September of that year. Moscow seemed to hope that internal divisions in China would distract Mao Zedong and Liu Shaoqi, the top leaders of China's Communist Party at the time.
Instead, China called international attention to the clashes and stared Russia down. And on Zhenbao Island, Chinese and Russian troops drew serious blood with 58 dead on the Russian side and 29 dead from China. So, that summer, highly placed Soviets, including the son-in-law of the Chairman of the Council of Ministers, began telling their counterparts in other nations that it might become necessary to take out China's growing atomic arsenal by force.
In April they said that, hey, maybe the best way to do that was with surgical nuclear strikes. It was the only way to restore the peace, after all.
China and Russia agreed to bilateral talks in 1970 that eventually restored peace, so it's possible that this was a bluff from the Soviet leaders. Maybe they believed that the threat of nuclear war could end the border clashes with no need to actually send any missiles or bombers up.
But it's also quite possible that the threat was real. While we in the West like to think of the Cold War as an all-consuming grapple between America and the Soviet Union, the Soviets were actually holding three times as many military exercises focused on their eastern border with China in the 1960s as they spent practicing for war with the U.S. and Europe.
So, yes, the world's first nuclear war could've been a clash between the Soviet Union and China, but that was thankfully averted. Unfortunately, China watched for weaknesses in the Soviet Union and, as the bloc started to crumble in the late 1980s, China made its move. While the Soviets tried to hold themselves together and America was preoccupied with finishing the fight and planning the post-Soviet world, China began an arms buildup.
And, uh, they've gotten stronger now. Including the nukes.