ANDAR DISTRICT, Afghanistan - Two Black Hawk helicopters swooped down into the makeshift base in the middle of a Taliban hotbed, and U.S. soldiers snapped salutes to the mission's top general — an Afghan, not an American. Afghan officers, with a big assist from U.S. counterparts, planned and executed their first major operation as part of a monthlong offensive, taking a key step forward in the U.S. effort to build up Afghanistan's army so it can secure the country on its own and let foreign troops leave. Operation Maiwand, which officially ended Tuesday, did not see heavy fighting. But Afghan...