In the 200 years since the American Revolution, the United States and Great Britain have moved from enemies to firm allies. War Plan Red, an American strategy to make war on Great Britain. It was conceived in the late 1920s and developed throughout the next decade, including the idea to build air bases capable of bombing cities in the British dominion of Canada, where the US top brass intended the conflict to take place. The plan was officially withdrawn in 1939, with the outbreak of the Second World War, although it was not declassified until 1974. Now, military experts and historians work through the plans as a war game, showing how they would have worked in practice and revealing who would have won if Great Britain’s closest ally had ever become its most dangerous enemy.