It was the Hitler’s attack on Soviet Union in 1941 that caused a big change in the entire situation of the II World War. It was on 21st June 1941, that fascist Germany attacked USSR. ON 22nd, Soviet Union declared all out defence offensive to the attack of Nazi Germany. Hitler thought that USSR could be defeated easily with the invincible war machinery Germany has got.
Germany advanced in to Soviet territory despite strong defence from USSR. It captured cities after cities and neared Moscow, the capital. USSR was fighting for its survival. It requested the UK, USA etc. to make a war front in the western side so that Hitler will have to fight on both sides. But they waited, hoping that USSR will be finished and the offensive can start after that.
But the people of Soviet Union and the Red Army were determined to throw out the aggressors at all costs.The defence production was increased many times, all the young men enrolled in the army, guerilla squads started active work in the areas captured by the Nazis and a fight to the last was started by the entire people. The pitched battle in Stalingrad, inch by inch, street by street, between the forces of Nazi Army and Red Army was an epic battle which will ever be remembered. The mighty army of Hitler was utterly defeated and thrown back. Thousands and thousands of soldiers perished on both sides. This was the turning point. The Red Army advanced destroying the war machine of Hitler. Nazi Army was routed. After throwing them from USSR, Red Army advanced to Berlin, captured the capital city and hoisted the Red Flag in the capital on 2nd May 1945.
When the tides turned in the war, UK and US started their advance on the Western Front. Their armies also reached Berlin after a few days. Hitler had already committed suicide by the time red army reached Berlin.