Despite loss in the Battle of Britain, despite abandonment of Sea Lion, despite set plans to create a two-front war, Adolph Hitler had Britain on her knees--because of the Mediterranean.
When Mussolini invaded Greece, Libya and Egypt in early 1941 Churchill rushed units to expel the Italians. This they did in quick order in utter embarrassment to Mussolini. Hitler felt a need, although he wished not to do it, focused as exclusively as he could be on Barbarossa, originally set to commence May 15, pushed back (fatally) to June 22 by a coup d'etat in Yugoslavia, to send a field general and a few divisions to North Africa in support of his friend. The field general was Erwin Rommel and it was he who more than any other individual turned a crushing Axis defeat into battlefield victories that nearly drove Britain from the war.
The refrain in Rule Britannia is "Britannia rules the waves." The British Fleet was the most powerful in the world. Britain's island security depended on ruling the waves, especially in the Mediterranean. But Hitler, an old infantryman, did not grasp the crucial importance of the Navy. It was, to him, an arm to pave the way open for the infantry.
Churchill was with Roosevelt when the cable came in: Tobruk had fallen. It was Churchill's darkest day of the war. Roosevelt saw physically on Churchill's face that something dreadful had befallen and when Churchill informed him of the contents of the message, replied with evident feeling, "How can we help?" Churchill would later tell Roosevelt that the loss of Egypt could reasonably mean Britain's discontinuation in the war.
Hitler did not infer as much. "Whether," it was possible to take Suez, which would have cut off that vital British supply route, it would have to wait, Hitler told Raeder, until after Barbarossa.
And so an opportunity was lost forever to defeat Great Britain and to make the coming great battle with the Soviet Union a one-front war. Hitler probably fatally endangered his own Russian plan as well. The coup in Yugoslavia "threw Adolph Hitler into one of the wildest rages of his entire life." (Shirer 824). Rage is almost always irrational, no? It was in this instance. Hitler took his eye off the ball and turned it menacingly on Yugoslavia. He utterly destroyed the country, but at a cost of four weeks delay in opening Barbarossa. The delay got Paulus stranded in Stalingrad in the depths of the Russian winter. All the German generals, including Paulus, were unanimous at Nuremberg that those four weeks cost them victory against Russia. And in the whole war had Hitler realized the state Britain was in in North Africa and finished the British off.