German operational theories were revised after the First World War. The Treaty of Versailles limited the Reichswehr to a maximum of 100,000 men, making impossible the deployment of mass armies. The German General Staff was abolished by the treaty but continued covertly as the Truppenamt (Troop Office), disguised as an administrative body. Committees of veteran staff officers were formed within the Truppenamt to evaluate 57 issues of the war. By the time of the Second World War, their reports had led to doctrinal and training publications, including H. Dv. 487, Führung und Gefecht der verbundenen Waffen (Command and Battle of the Combined Arms), known as das Fug (1921–23) and Truppenführung (1933–34), containing standard procedures for combined-arms warfare. The Reichswehr was influenced by its analysis of pre-war German military thought, in particular infiltration tactics, which at the end of the war had seen some breakthroughs on the Western Front and the manoeuvre warfare which dominated the Eastern Front.
Conventional wisdom traces blitzkrieg, “lightning war,” to the development in Germany between 1918 and 1939 of a body of doctrine using mobility to prevent repetition of the attritional deadlock of World War I. Soldiers such as Hans von Seeckt and Heinz Guderian allegedly perceived more clearly than their counterparts elsewhere in Europe the military potential of the internal-combustion engine combined with modern communications technology. Large formations moving on tracks and wheels, directed by radios, could rupture an enemy’s front and so disorganize its rear that countermeasures would be paralyzed. First tested in Poland, the concept reached perihelion in France and the Low Countries in 1940, when in less than six weeks the German army crushed the combined forces of four nations. Applied a year later against the Soviet Union, blitzkrieg purportedly brought the Wehrmacht to the gates of Moscow in six months. Some accounts insist that only Adolf Hitler’s incompetent interference tipped the war’s balance so far against Germany that even blitzkrieg’s most sophisticated refinements could do no more than stave off the Reich’s collapse.
Schwerpunktprinzip was a heuristic device (conceptual tool or thinking formula) used in the German army since the nineteenth century, to make decisions from tactics to strategy about priority. Schwerpunkt has been translated as centre of gravity, crucial, focal point and point of main effort. None of these forms is sufficient to describe the universal importance of the term and the concept of Schwerpunktprinzip. Every unit in the army, from the company to the supreme command, decided on a Schwerpunkt through schwerpunktbildung, as did the support services, which meant that commanders always knew what was most important and why. The German army was trained to support the Schwerpunkt, even when risks had to be taken elsewhere to support the point of main effort. Through Schwerpunktbildung, the German army could achieve superiority at the Schwerpunkt, whether attacking or defending, to turn local success at the Schwerpunkt into the progressive disorganisation of the opposing force, creating more opportunities to exploit this advantage, even if numerically and strategically inferior in general. In the 1930s, Guderian summarised this as "Klotzen, nicht kleckern!" ("Kick, don't spatter them!").
Even before the Nazis came to power in Germany in 1933, they had made no secret of their anti-Semitism. As early as 1919 Adolf Hitler had written, “Rational anti-Semitism, however, must lead to systematic legal opposition.…Its final objective must unswervingly be the removal of the Jews altogether.” In Mein Kampf (“My Struggle”; 1925–27), Hitler further developed the idea of the Jews as an evil race struggling for world domination. Nazi anti-Semitism was rooted in religious anti-Semitism and enhanced by political anti-Semitism. To this the Nazis added a further dimension: racial anti-Semitism. Nazi racial ideology characterized the Jews as Untermenschen (German: “subhumans”). The Nazis portrayed the Jews as a race and not as a religious group. Religious anti-Semitism could be resolved by conversion, political anti-Semitism by expulsion. Ultimately, the logic of Nazi racial anti-Semitism led to annihilation.
Uniquely at Auschwitz, prisoners were tattooed with a serial number, on their left breast for Soviet prisoners of war and on the left arm for civilians. Categories of prisoner were distinguishable by triangular pieces of cloth (German: Winkel) sewn onto on their jackets below their prisoner number. Political prisoners (Schutzhäftlinge or Sch), mostly Poles, had a red triangle, while criminals (Berufsverbrecher or BV) were mostly German and wore green. Asocial prisoners (Asoziale or Aso), which included vagrants, prostitutes and the Roma, wore black. Purple was for Jehovah's Witnesses (Internationale Bibelforscher-Vereinigung or IBV)'s and pink for gay men, who were mostly German. An estimated 5,000–15,000 gay men prosecuted under German Penal Code Section 175 (proscribing sexual acts between men) were detained in concentration camps, of which an unknown number were sent to Auschwitz. Jews wore a yellow badge, the shape of the Star of David, overlaid by a second triangle if they also belonged to a second category. The nationality of the inmate was indicated by a letter stitched onto the cloth. A racial hierarchy existed, with German prisoners at the top. Next were non-Jewish prisoners from other countries. Jewish prisoners were at the bottom.
Despite the continuing war with Great Britain, German forces invaded the Soviet Union in June 1941. At first, the German Blitzkrieg seemed to succeed. Soviet forces were driven back more than 600 miles to the gates of Moscow, with staggering losses. In December 1941, Hitler unilaterally declared war on the United States, which consequently added its tremendous economic and military power to the coalition arrayed against him. A second German offensive against the Soviet Union in 1942 brought German forces in the east to the shores of the Volga River and the city of Stalingrad. However, the Soviet Union launched a counteroffensive in November 1942, trapping and destroying an entire German army at Stalingrad.
Of the 430,000 sent to the first death camp at Bełżec in Poland, there were only two survivors. 700,000 were killed at Treblinka in just five months. In July, Himmler ordered that all Jews in key areas of Poland, except for those needed for essential labour, were to be killed by the end of the year. Most were. Despite Allied intelligence receiving detailed reports of the mass murders in Europe, the public reaction in Britain was largely a mixture of apathy and disbelief.
The Germans invaded the Netherlands, Luxembourg, Belgium, and France in May 1940. In the Netherlands, the Germans installed Arthur Seyss-Inquart as Reichskommissar, who quickly began to persecute the approximately 140,000 Dutch Jews. Jews were forced out of their jobs and had to register with the government. Non-Jewish Dutch citizens protested these measures, and in February 1941 they staged a strike that was quickly crushed. After Belgium's surrender at the end of May 1940, it was ruled by a German military governor, Alexander von Falkenhausen, who enacted anti-Jewish measures against the country's 90,000 Jews, many of whom were refugees from Germany or Eastern Europe.