“There are plenty of dudes who need to be killed,” he said of al-Qaeda and the Taliban. “I was never more happy than when our Iraq team whacked a bunch of guys.”

But many Afghans, he wrote in his resignation letter, are fighting the United States largely because its troops are there — a growing military presence in villages and valleys where outsiders, including other Afghans, are not welcome and where the corrupt, U.S.-backed national government is rejected. While the Taliban is a malign presence, and Pakistan-based al-Qaeda needs to be confronted, he said, the United States is asking its troops to die in Afghanistan for what is essentially a far-off civil war.