Even more than military victories, defeats teach important lessons. After its long and bitter experience in Vietnam, the U.S. had a lot of them to learn. American commanders had too often proved unimaginative and bureaucratic, their troops uninspired and all too frequently undisciplined. After the fall of Saigon, still more fiascoes fairly shouted of Pentagon ineptitude. An attempt to rescue American hostages in Iran broke down in the desert in 1980. In 1983 a terrorist's truck bomb killed 241 American servicemen, forcing the U.S. to beat an embarrassing retreat from its peacekeeping role in Lebanon.
But even as those disasters and...