On June 22, 1941, Germany launched its invasion of the Soviet Union during World War II, codenamed Operation Barbarossa. Nazi leader Adolf Hitler predicted a quick victory, but after initial success, the brutal campaign dragged on and eventually failed due to strategic blunders and harsh winter weather, as well as a determined Soviet resistance and attrition suffered by German forces. The failure of Operation Barbarossa reversed the fortunes of Nazi Germany. It is became one of the biggest cause of German loss in WW2 and also the invasion of USSR proved biggest mistake of Adolf Hitler in leading WW2.