In my mind the moment when Hitler decided to wage war on the Soviet Union, that was the moment Germany lost the war. The dreaded two front war. Warfare changes in an urban environment, Lenningrad, Moscow, and Stalingrad were threatened, but never taken by the Germans
Unfortunately, the French didn't fight for Paris, allowing the bulk of its army to be surrounded by the Germans further east and north.
Britian did avoid a German invasion in summer of 1940 during the Battle for Britain. Yes, it was a close run adventure, but the Germans blinked first. However, the war was not going well for Britian in the fall of 1941. Britian was hanging on.
In that time frame of late 1941 America was isolationist, although being the arsenal of democracy. Its welcomed lend lease shipments would not have won the war for democracy, only its armed forces would swing the outcome. But after our experience with the First World War, America did not wish to get involved into another European war, much less an Asian war.
Fortunately for the free world, America had strategic thinkers in charge of its armed forces. Marshall, Eisenhower, MacArthur, Arnold, King, and Nimitz. All of them earned their fifth star.
But the bottom line is the Russians swung the tide of their war with Germany during the winter of 1942-43 at Stalingrad. After Germany left Britian to hang on a limb, it decided its own fate waging war with the Russians. Therefore, its not a matter whether Germany could defeat Britian, not after Russia defeated Germany.