I will concur with the majority that have answered this question before me; Hitler's decision to turn against Russia prior to defeating the British was a major turning point in WW2. Diplomatically he had bought off the Russians with the invasion and "sharing/splitting" of Poland in 1939. You figure he would have used this to allow him enough time to secure his western front. Fortunately for the world he did not do this.
(Perhaps if Hitler were ever to appear reincarnated as a household cat we might be able to ask him, why he turned his attention on Russia when he did?)
On another note, Ike's decision not to fuel Patton's astonishing advances in 1944 surely extended the war in Europe by months and allowed for the Soviets to be in a much stronger position in Eastern Europe by 1945. The consequences of this of course were felt during the Cold War.
Hindsight is 20/20 I suppose.