OK - here goes.
May 1940. Hitler smahes the BEF on the shores of Dunkirk. Very few get away. Churchill, stunned by this, journeys to Dover to meet with the few survivors, and is killed by a deranged soldier with too much combat fatigue.
Without his leadership, the government falls, and is replaced by (ASB intervention required) Oswald Moseley. He pulls Britain out of the war after France throws in the towel. To further consolidate his hold on government, he declares the King a traitor and arrests him - however Princess Elizabeth escapes and flees to Canada.
Officially, the war ends. But the young Princess is roaring defiance from Ottawa. She meets and quite charms Roosevelt (similar to that Norwegian Princess in OTL). With people fleeing Fascist Britain, the other commonwealth states (sans South Africa) meet in Ottawa and crown Elizabeth empress on Jan 1, 1941.
Moseley, incenced, declares war, and orders the Royal Navy to sail. Hitler - caught up in Barbarossa preparations - shrugs and says who cares? The RN is augmented by the ships of the Kriegsmarine (including the BISMARCK) and the odd Italian - Moseley promises then aid against Russia for help.
At this point, the young Empress makes a heartfelt and poignant address to a joint session of Congress, where she invites the United States to join in the Commonwealth. The US will recognize her as the 'Commonwealth of Anglosphere Nations' Empress, however she will have no power over US domestic or foriegn policy making, and agrees that the US President will also be 'Prime Minister' of the Commonwealth. Congress agrees, and the US Navy sorties against the oncming British Italian and German fleets...
Mike Turcotte