The downfall of Germany in the Second World War started with the invasion of the Soviet Union in June 1941. After some initial successes, the German Army suffered heavy defeats against the Soviet Army in the harsh Russian winter. The German Army suffered its heaviest defeat in the Battle of Stalingrad in February 1943.
The attack of Japan on the US naval base of Pearl Harbour in December 1941 prompted the US to enter the war against the Axis powers. The US entered the war in Europe against Germany with the Normandy invasion in June 1944. From then, Germany faced a two front war with the Soviet Union attacking from the East and the US and Britain from the West.
The Germans had no hope of winning a two front war. They gradually started losing the territories they had conquered in both Eastern and Western Europe. They did launch a final offensive in the Ardennes mountains of Western Europe but lost that battle as well.
With his empire collapsing, Hitler retreated to his bunker in Berlin to launch a final defence of the capital. But this was a futile campaign. The Soviet Army entered Berlin in April 1945. Seeing no hope, Hitler committed suicide on April 30, 1945. Germany then surrendered on May 8, 1945.