When the United States invaded Afghanistan after the attacks on September 11, 2001, and then overthrew the Taliban regime, senior military officers were not predicting that the United States would be militarily involved 18 years later. Yet, after expending nearly $800 billion and suffering over 2,400 killed, the United States is still there, having achieved at best a stalemate. This CSIS report concludes that the mission in Afghanistan expanded from a limited focus on counterterrorism to a broad nation-building effort without discussions about the implications for the duration and intensity of the military campaign. This expansion occurred without considering the history of Afghanistan, the Soviet experience, and the decades-long effort required in successful nation-building efforts. The report makes a series of recommendations: improving the dialogue between senior military and civilian officials about desired goals/end states and the implied intensity/duration of a military campaign; continuing the development of military strategists; revising military doctrine publications to include discussion of choices about goals/end states; and taking more seriously the history and experience of others.