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This question is inspired by the recent movie, but also by Frieser's book about the Battle of France. So let's assume that Hitler never gives the halt order. Instead of being evacuated, the majority of the British troops are taken prisoner by the Germans. This means that around 300,000 soldiers of the BEF are now prisoners of war. According to Frieser, this would have led to Churchill's resignation and to an armistice, because Britain would have been unable to continue the war without its professional army.
But I don't want to explore this option. Maybe Churchill somehow manages to stay in office; maybe Halifax succeeds him as Prime Minister, but no armistice is concluded due to excessive German demands.
So how would WWII go on if the British army is destroyed at Dunkirk? How long would it take until Britain is re-formed an army capable of large-scale operations? And how would the situations on other fronts develop, like in North Africa or in the East Indies (once the Japanese start their exansion)?