It's interesting to think about the counterfactual of what would have happened in Europe if Britain had decided not to defend Poland. France would probably have followed Britain's lead, as she did after Munich in 1938. There would have been no need for a Hitler-Stalin Pact, and it wouldn't have mattered anyway. Germany would have conquered Poland easily in 1939. In 1940, Russia would not have dared to absorb the Baltic republics and Bessarabia, and would not have had the part of Poland it conquered in actual history. Germany could have attacked then instead of 1941, and without fiddling around in the Balkans first. The Russian army would have been one year less far away from Stalin's purge of most of its generals in the 30s. Presumably Germany would have done even better than it did in 1941. Britain, France, and the US would not have aided Russia-- it would be hard to defend helping Stalin when they weren't fighting a common war with him. Japan was engaged in tank battles with Russia in real history in 1940, and with the best Russian troops sent to fight Hitler in my alternative history, the Japanese would have attacked vigorously. This Northern strategy would clearly have dominated going South to Indochina, the Philippines, and the Dutch East Indies, so there'd be no reason for America to get involved. If Russia didn't lose in 1940, it would lose in 1941.
What next? Hitler might have settled down to organizing the Slavs for a few years, or he might have gone straight to conquering France and the Balkans. He couldn't have beaten the Royal Navy and Air Force in 1940 or 1941, but he could have by, say, 1950. Whether he'd want to is not entirely clear, but I think he'd like the idea of world domination. He might have tolerated an independent Britain if it refrained from electing a Labor government.