This book starts by noting about the empirical evidence and research done by Dupuy and others about the fighting power of the German army. He notes that in engagements in 1943 and 1944 the German army inflicted about 50% more casualties per soldier engaged than the allies, in other words, that the German forces were much more effective fighting organizations than allied formations. The author sets the question: How they managed to maintain such superior fighting power? And uses the rest of the book to gather evidence.
He compares the status of the army to the rest of the society, and shows that the German army enjoyed a higher social status than the American army, that meant that the German army could draw over the cream of society, while the US army drew mostly from the lower/less educated classes. Also, selection processes were more strict in the German armed forces than in the American, selecting only the most apt for the job. And even in 1944, the basic training of the German soldier was a bit longer than for the American soldier.
The problems of the US replacement system are analyzed: The US army replaced soldiers individually, putting men in the middle of the battle without any training experience with the soldiers in his unit. This resulted from ignoring the important psychological aspect of war. As result, American divisions with a lot of replacements started suffering higher casualties than fresher units. The German divisions by contrast, replaced their men by battalions of 500 men that trained together and went together to the front.
The differences also involved the focus of the respective armed forces: While the Germans focused on operations the American doctrine focused on the logistics of maintaining material superiority. Since the US was the largest industrial economy in the world, they could always count on their superior material resources for any conflict that they participated. This meant that their military organization focused on coordinating these vast material resources into an effective front.
The conclusion is that the Germans developed such high fighting power because of their needs: Always facing powerful foes in their borders (France and Russia), and in the event of a major war, always under the risk of severe numerical inferiority (with happened in both world wars) in the event of a multi-front war, they had to make more with less: They had to maximize the military effectiveness of their resources to compensate for their lack of material superiority. The Americans in the other hand, without any powerful neighbors (Canada and Mexico) and traditionally isolated from the rest of the world, hardly maintained a level of military mobilization near the level of European powers and in the case of war, always fought on the side with superior resources (both in terms of men and 'materiel'). Hence, their military never had to develop fighting power: They didn't need to maximize the effectiveness of their military resources, since they had always several times more resources than the enemy.