Unlike the American 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 West (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.