The story begins in 1924 when a young politician named Oswald Mosley is elected in the Birmingham general election against Neville Chamberlain, continuing to become an important figure in British politics. However, in 1929, the Great Depression worsens, causing the Democratic Party to form an alliance with the Rural Labour Party and the Socialist Party in 1932 to launch U pton Sinclair for president, who defeats Herbert Hoover by a small margin and becomes president.

In 1936, when King Edward VII married Wallis Simpson and refused to abdicate the throne, Prime Minister Baldwin resigned and Mosley became the new Prime Minister. This left the United States very concerned that one of its former great allies would become a rival. In 1937, after being re-elected following a vote in Congress, Sinclair invaded Canada. 

After the US made some advances in Brunswick and Nova Scotia and with the conquest of Toronto, trench warfare began until, in 1938, several protests and strikes began to take place throughout the US, with the UK managing to expel the US from Canada and with the arrival of volunteers from several European countries such as Germany and Italy (in this universe, Germany tested its army in the US rather than in Spain, even sending some SS divisions to fight) and with Mexico also invading to try to recover its former territories, but with little success, only managing to conquer parts of Arizona and New Mexico. In 1941, the final nail in the coffin for the US defeat was hammered in when the South rebelled against Sinclair for his various defeats and also for having run for and won the 1940 elections and for his attempt to abolish segregation. the front in the North collapsing and with the fall of Washington DC, Sinclair flees to California while the US Army, which has taken control of the Midwest, signs a treaty with the United Kingdom handing over the regions of Cascadia, the Great Lakes, and New England.

With the United Kingdom creating the Commonwealth of Cascadia led by William Dudley Pelley and the Commonwealth of Columbia led by William Bell Riley, with Canada annexing New England, with the German invasion of Poland in 1939 and the United Kingdom paying little attention due to its war with the US, and with the Germans bogged down in a war against the Soviet Union in 1941, having already conquered almost all of Eastern Europe, Mosley considers invading Germany for its ultra-expansionist actions, which may one day lead to a war with the United Kingdom.

What If the US launched War plan Red and Lost?