Once the Soviet Union entered the war, the Nazis were surrounded and faced a two-front conflict with Western European allies and the US to the west and the Soviet Union to the east.
I believe that when the Soviet Union entered the war the Nazis had just moved into Russia during winter so they not only had to stay alive during their battles they now had to fight the cold to stay alive. It was much harder for them to get supplies since they were so far into Russia when it started snowing.
