KABUL, Afghanistan — A man dressed in an Afghan army uniform opened fire Tuesday on NATO troops at a military base, killing a U.S. two-star general and wounding some 15 people, among them a German brigadier general and a number of Americans troops, authorities said.

The attack at Camp Qargha, a base west of the capital, Kabul, killed who is believed to be the highest-ranking U.S. officer to die in the nearly 13-year war and comes as foreign troops prepare to withdraw by the year’s end. While details remained murky about what sparked the attack, it showed the challenges still remaining in Afghanistan, a nation that’s known three decades of war without end.