When U.S. forces and their Afghan allies rode into Kabul in November 2001 they were greeted as liberators. But after 17 years of war, the Taliban have retaken half the country, security is worse than it’s ever been, and many Afghans place the blame squarely on the Americans. The United States has lost more than 2,400 soldiers in its longest war, and has spent more than $900 billion on everything from military operations to the construction of roads, bridges and power plants. Three U.S. presidents have pledged to bring peace to Afghanistan, either by adding or withdrawing troops, by engaging...