Hermann Goering, Luftwaffe leader and one of Hitler's right hand men, was largely responsible for Nazi Germany's defeat in the Second World War. After the German rout of French and British forces during the bliztkreig of France, German panzers were ready to roll over British forces left in the port of Dunkirk. Goering promised Hitler that his aircraft would be able to bomb them into the ground. Hitler took the advice of his friend, but despite the aerial barrage the British scrounged together a fleet of private and military ships that evacuated thousands of soldiers to the British mainland. If these soldiers had been killed or captured, the British would have had virtually no army and would probably have been forced into an armistice or face invasion.
During the Battle of Britain Goering's Luftwaffe was starting to gain the upperhand when Goering stepped in and lost it. Originally the Luftwaffe was bombing British coastal radar stations and airfields. The bombing of airfields was taking a terrible toll on the pilots and aircraft. At a critical point of the battle Goering abruptly switched tactics and began bombing British cities. While it did take an emotional toll on the British people, it allowed time for British industry to manufacture new aircraft, and for the RAF to train new pilots. Eventually this led to a second chance lost to force the British into an armistace.
His third mistake took place on the Eastern Front with the fight against the Soviet Union. The Battle of Stalingrad was raging, and it was becoming clear that the Germans were losing. Goering promised Hitler during the winter that he could supply General Paulus's army. This was in part responsible for Hitler's decision to prevent a strategic withdrawl which would have saved the army and probably allowed a victory against the Soviet Union. Instead, Paulus's army was encircled and forced to surrender when Goering's promise was not met (once again) and General Zhukov began the long drive to Berlin and the final defeat of Nazi Germany due to the collapse of a major German presence on the Eastern Front.