In 1939, Germany and the USSR follow their original Molotov-Ribbentrop borders, with the Polish border being slightly further west and Lithuania being a German puppet. The USSR creates a new Polish SSR. The Fall of France goes the way it did in OTL. When it comes time for Operation Barbarossa, it's even more devastating than in our timeline due to Lithuania being puppeted, and the USSR is in even worse shape by the end of 1941, although they hold on.
Meanwhile, Germany evacuates their 3,000 fighters from North Africa in 1943 instead of reinforcing it, making the air war more deadly for the allies. But here's the real kicker: The Germans unleash nerve gas on the allied forces on both fronts. They had a large lead in nerve gas compared to the allies. The Germans also invest heavily in anti-air missiles and jet fighters, and begin to field them in early 1944, making the air war even deadlier.
Despite the USSR being in a much worse position, they and the Wallies still begin to crush the Reich, but the war is much deadlier than in OTL. The Normandy invasion is a lot bloodier, and V1 and V2 rockets start hitting Britain armed with nerve gas.
Germany holds out until August 1945, when the nukes start falling on them. The allies have been dropping anthrax and other chemical weapons on Germany for years now, and the war is just generally much nastier. The nukes being used on Germany means that the invasion of Japan goes ahead. The USSR is in Berlin by January 1946, and they go on even more of a rampage than in OTL.
All in all, the war is just much bloodier for everyone involved, and Germany is utterly broken after the war. The allies are determined to punish them for their crimes.