Our perception of land operations in the Second World War has...been distorted by an excessive emphasis upon the hardware employed. The main focus of attention has been the tank and the formations that employed it, most notably the (German) panzer divisions. Despite the fact that only 40 of the 520 German divisions that saw combat were panzer divisions (there were also an extra 24 motorised/panzergrenadier divisions), the history of German operations has consistently almost exclusively been written largely in terms of blitzkrieg and has concentrated almost exclusively upon the exploits of the mechanized formations. Even more misleadingly, this presentation of ground combat as a largely armored confrontation has been extended to cover Allied operations, so that in the popular imagination the exploits of the British and Commonwealth Armies, with only 11 armored divisions out of 73 (that saw combat), and of the Americans in Europe, with only 16 out of 59, are typified by tanks sweeping around the Western Desert or trying to keep up with Patton in the race through Sicily and across northern France. Of course, these armored forces did play a somewhat more important role in operations than the simple proportions might indicate, but it still has to be stressed that they in no way dominated the battlefield or precipitated the evolution of completely new modes of warfare.
Throughout the spring and summer of 1940, the German army expanded Hitler’s empire in Europe, conquering Denmark, Norway, the Netherlands, Belgium, Luxembourg and France. Beginning in 1941, Jews from all over the continent, as well as hundreds of thousands of European Gypsies, were transported to the Polish ghettoes. The German invasion of the Soviet Union in June 1941 marked a new level of brutality in warfare. Mobile killing units called Einsatzgruppenwould murder more than 500,000 Soviet Jews and others (usually by shooting) over the course of the German occupation.
The influence of air forces over forces on the ground changed significantly over the course of the Second World War. Early German successes were conducted when Allied aircraft could not make a significant impact on the battlefield. In May 1940, there was near parity in numbers of aircraft between the Luftwaffe and the Allies, but the Luftwaffe had been developed to support Germany's ground forces, had liaison officers with the mobile formations, and operated a higher number of sorties per aircraft. In addition, German air parity or superiority allowed the unencumbered movement of ground forces, their unhindered assembly into concentrated attack formations, aerial reconnaissance, aerial resupply of fast moving formations and close air support at the point of attack. The Allied air forces had no close air support aircraft, training or doctrine. The Allies flew 434 French and 160 British sorties a day but methods of attacking ground targets had yet to be developed; therefore Allied aircraft caused negligible damage. Against these 600 sorties the Luftwaffe on average flew 1,500 sorties a day. On May 13, Fliegerkorps VIII flew 1,000 sorties in support of the crossing of the Meuse. The following day the Allies made repeated attempts to destroy the German pontoon bridges, but German fighter aircraft, ground fire and Luftwaffe flak batteries with the panzer forces destroyed 56 percent of the attacking Allied aircraft while the bridges remained intact.
By the spring of 1942, the Nazis had established six killing centers (death camps) in Poland: Chelmno (Kulmhof), Belzec, Sobibor, Treblinka, Maidanek and Auschwitz. All were located near railway lines so that Jews could be easily transported daily. A vast system of camps (called Lagersystem) supported the death camps. The purpose of these camps varied: some were slave labor camps, some transit camps, others concentration camps and their subcamps, and still others the notorious death camps. Some camps combined all of these functions or a few of them. All the camps were intolerably brutal.