Eastern Front and Western Front
In my opinion, the Soviet Union alone would have been enough to win the war.
Alessandro Barbero, Past and present
The Eastern front
For the invasion of the USSR, the largest invasion forces ever seen were prepared: 3.5 million men (70% of the men of the Wehrmacht), 146 divisions of which 19 were armored. On the eastern front the three best armored divisions of the Waffen SS were employed: the "Das Reich", the "Adolf Hitler" and the "Totenkopf" as well as the elite Panzergrenadier division "Grossdeutschland" equipped with the best armored vehicles.
80% of German battle casualties and 2/3 of the losses of German tanks and guns of WW2 occurred on the Eastern Front.
"While the defeat of German forces was the key strategic task, the main theater was the conflict on the Eastern Front, where German forces were weakened and then pushed back, before Allied air and ground forces made their mark weight in 1944. More than four hundred German and Soviet divisions clashed on a front of more than a thousand miles.Soviet troops destroyed or disabled some 607 Axis divisions between 1941 and 1945. Size and geographical extent of the Eastern Front surpassed any previous wartime episode. Casualties on both sides far exceeded those in any other combat zone during a military conflict. The war in the East was fought with a ferocity almost unheard of on the Western Fronts."
Richard Overy - Why the allies won and The Russia's war
The Western Front
The other major fronts involved much smaller forces for most of the war. The German Army never fielded more than twenty divisions in the Mediterranean theater, but it managed to prevent Allied victory in that area for four years. France in 1944, where Germany would commit more than fifty divisions, most incomplete and some badly armed, in its decisive phase was fought between fifteen Allied and fifteen German divisions.Throughout the course of the conflict the German forces used a greater number of tanks, cannons and tactical planes in the east than on the other fronts."
Richard Overy - Why the allies won, 1995
“The Germans could easily withdraw 15-20 divisions from the Eastern Front to help their Western Front, but this is not the case. They fight furiously in the east for a dark station, as useful as a poultice of herbs to a dead man, but surrender Mannheim and Kassel without firing a shot...strange and inexplicable behaviour.”
Joseph Stalin - April 1945
“The OKW usually transferred troops in need of rest and supplies from the Eastern to the Western Front, after which, the gaps filled and rearmed, they were sent back to Russia. This is why our tables did not reflect the true situation. They usually included divisions which in fact were left without arms and without personnel. But the shifting of units continued.”
General B.Zimmerman - France in 1944 in Fatal Decisions p.219
« In the western theater of operations most German soldiers are too old. Officers with artificial limbs were often on duty. A whole battalion was made up of men affected by hearing diseases. Later, the entire 70th Division was filled with soldiers who had stomach ailments and needed a special diet… There was a serious lack of heavy weapons, especially tanks.”
Field Marshal Gerd von Rundstedt - Commander-in-Chief of Army Group West, - report submitted to the OKW, 1944
"Many Western historians of the Second World War place the Battle of Stalingrad on the same level as the British offensive from El Alamein to Tunis although in their importance they are not comparable. It is known that only four German divisions were operating on the North African front and eight Italians."
A.M. Samsonov - Stalingrad