Hitler has already controlled most Western-Central Europe when he decided also to fight against the URSS on the Eastern front. By that time, Germany had already conquered almost the whole France, Austria, Belgium, the Netherlands, Poland, Czecoslovakia, etc.
The URSS and nazi Germany had signed a non-agression pact before WWII. Germany had conquered territories on the East until a Germany-URSS border was established. When Hitler decided to attack this Eastern border, the URSS abandonned its neutral position and joined the allied forces against Hitler. The allies benefited greatly from having on their side such a powerful help.
From this moment on, also due to the internvention of the US on the conflict, US and UK fought Germany on the Western Front while the URSS did the same on the Eastern front. Germany had to divide its forces between two very powerful enemies located on each front and this led to its destruction.