Unlike the United States Army, the German Wehrmacht had a long standing professional officer corps that had experience going back to the Franco-Prussian War of 1871. While many American career officers had seen action in World War I, the vast majority of Americans entered combat for the first time.
Secretly, the Nazis built the 100,000 men allowed under the Versailles Treaty — for internal security — into a highly trained officer corps. The Night of Long Knives in 1934 ensured Army loyalty by removing the SA and its leader, Ernst Röhm.
By the eve of the Polish Campaign, the Wehrmacht had 2,000,000 men. The tactics that Heinz Guderian and others invented — Blitzkrieg — smashed through Poland, and then France and the Low Countries, Denmark, and much of Russia.
The Germans had enveloped themselves in precisely the situation Hitler warned about in Mein Kampf. Dividing their forces under two commands, Oberkommando des Heeres (OKH) and Oberkommando des Wehrmacht (OKW.) The majority of men and materiel were sent to the Russian Front, duty detested by the regular army troops.
Casualties in the West had been very light, but during Operation Barbarossa thousands and then hundreds of thousands were killed. Nevertheless, Wehrmacht morale remained high, due in part to the Hitler Youth program, which placed emphasis on nationalistic ideals and group loyalty; personal loyalty to Hitler was above all. The Wehrmacht gained a reputation as an unbeatable foe, and the endurance of the German soldier was legendary. The stereotype was so powerful that even in 1944 some Allied troops feared attacking German units without total numerical supremacy.
The centralization of authority was also a fatal flaw in German command doctrine. Unlike the independent authority Allied unit commanders and even noncommissioned officers had to alter or even abandon operational plans in the face of tactical needs, the German High Command often limited their commanders by requiring personal permission from Hitler or other superiors in order to gain needed units or supplies. The Allies could often adapt to changing battlefield tactics, while the German officers could not.
In the orgy of violence that was perpetuated by the Nazis, the Gestapo or SS units like the Einsatzgruppen committed many atrocities. The Wehrmacht was not blameless; except for North Africa, were Generalfeldmarschall Erwin Rommel personally forbade reprisals, Germany Regular Army units shot and killed civilians and POWs in every theatre. The Wehrmacht was especially vicious in the Eastern Front.
Hitler, disdainful of the professional soldier, often overruled his commanders and sacked them for spurious reasons. After the July 20 plot to assassinate him, tens of thousands were executed or forced to commit suicide, including the able Rommel.
In 1944 the huge manpower losses forced the drafting of thousands of young and old men into Volksgrenadier divisions that were under strength but had additional automatic weapons and Panzerfaust antitank weapons. They were the only men left available to replace the huge losses at Stalingrad, North Africa and elsewhere. Even the SS, which required applicants to prove they had Aryan lineage before joining, accepted men from all over Europe by the end of the war.
In the end, with constant air attack, huge armies with superior technology on both sides, the professional army of Germany tried to defend her own borders. But without the ability to produce enough equipment, with millions dead, the Wehrmacht ceased to be the vaunted fighting force of legend.