What was the main thing that USA failed to learn after WWII and is costing us so dearly to this very day?

In their work, Junke and Hunter have explained that Kennan had very often shared that almost unequivocally, "Americans who expected the full fruits of total victory [in the late 1940s]" had failed to understand the war. It was one case of a wider phenomenon, the 'failure to appreciate the limitations of war in general--any war--as a vehicle for the achievement of the objectives of the democratic state.'"