In 1944, Leslie Groves, the US army general who managed the Manhattan Project, asked its chief scientist, J. Robert Oppenheimer, just how powerful their new bomb might be. Would it be 10 times as powerful as the largest bomb of the time, the RAF’s Tallboy “earthquake bomb”? Or 50 times, or even 100 times? Oppenheimer replied that he could not be sure — at the time, there were even fears that the explosive chain reaction might never stop — but he expected a bomb much more powerful than 100 Tallboys. Groves immediately replied that such a powerful weapon would not be of much use to anyone, because the “politicians” would never dare to use it.

In the short run, Groves was wrong, while Oppenheimer’s guess was correct. The Hiroshima uranium bomb was in fact more powerful than 1,000 Tallboys, with the Nagasaki plutonium bomb exceeding even that. But only five years later, Groves’s prediction came to pass. First the United States, and then the Soviet Union, and then each successive nuclear power came to the realisation that their nuclear weapons were too powerful to be used in combat. This has remained true in the decades since — all the way up to the invasion of Ukraine. For, despite Putin’s atomic sabre-rattling, he too is subject to the logic of Groves’s prediction. Decades after his conversation with Oppenheimer, a brief historical summary of nuclear war has much to teach us about the situation in Ukraine — and how victory might only be attainable there through much more conventional means.

The first test of the nuclear age came with the Korean War. In December 1950, hundreds of thousands of Chinese soldiers crossed the River Yalu to support their North Korean allies against the US. With America in immediate danger of losing tens of thousands of men, General Douglas MacArthur decided that he had to use nuclear weapons to stop the Chinese. By far the most respected US military leader of the time — he had led American forces in the Pacific from humiliating defeat to total victory, and then acted as Japan’s de facto Emperor in reforming the country — MacArthur expected Truman to assent to his superior military judgment. Instead, the answer was a flat no. MacArthur insisted, and he was dismissed.

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