War Plan Red was an American strategy conceived in the late 1920s aimed at launching an attack on Great Britain. 

In fact, a budget of £57m was set aside to War Plan Red into practice. The ideas was to build air bases disguised as airports capable of attacking cities in the British dominion of Canada, where the US top brass intended the conflict to take place.

Bombing raids on industries, chemical warfare and naval blockades were all part of the strategy designed to diminish Britain's status as a worldwide power and cutting off vital trading routes was the key to this plan. 

This antipathy towards Britain dated back to the American War of Independence and following WWI the US was owed $14bn by the UK.

It was officially withdrawn in 1939, following the outbreak of the Second World War, although it was not declassified until 1974. 

In this one-hour documentary, military experts and historians talk 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.