It's tough to answer this accurately and fully without putting the whole thing in perspective. For one thing, halfway through the war, the allies still had not opened up a western front against Germany per Stalin's requests. It was only after Stalin no longer needed a western front that the allies finally invaded Normandy--the German war ended 10 months later. The war in Europe was primarily a war between Nazi Germany and Communist Soviet Union, even though Germany and Britain were at war before Germany attacked the Soviets. In the build-up to war, Hitler annexed all of the countries that had formerly been part of Germany in order to make Germany whole again. The Soviet Union did the same thing with Latvia, Lithuania, and Estonia. When Germany invaded Poland to make a buffer state from which to eventually invade Russia, the UK and France declared war and interfered with his plans for a war against the Soviets. The Soviet Union then took eastern Poland and tried to take over Finland. Hitler and Stalin made peace, but both knew war would eventually come. Once France was defeated and Britain was in a defensive posture, Hitler felt safe enough to finally invade the Soviet Union. The rest of the battles in the west had to do with Germany "bailing out" its ally, Italy, in Italy's own separate plans for expansion. The Italians tried to conquer Egypt from Italian-owned Libya, but failed and needed Germany's help. After occupying Albania, Italy failed again in its attempt to conquer Greece and Crete and needed Germany's help again. After the US joined the allies in the west and after Italy surrendered, western Europe became an irritating distraction for Germany. On the Eastern front, Germany placed the bulk of its forces with a huge allied contingent of Romanians, Hungarians, Austrians, Italians, Bulgarians, and even Spaniards. Right up to the end, the German leadership deluded itself into thinking that it could make peace with the UK and US since the Nazis formed a buffer state against the hated Communists. This idea was shared and somewhat popular among many US military leaders, who preferred the Soviets as an enemy instead of the Nazis.