WASHINGTON — The attack on a U.S. diplomatic outpost in Libya last year has become a factor driving the White House decision on how large a force to leave in Afghanistan after 2014 — and a specter hanging over talks between the Afghan president and the U.S.

Afghan President Hamid Karzai has publicly called for a near-total drawdown of U.S. forces, with a surge of U.S. and international aid to make up for their exit.

But after losing a U.S. ambassador and three other Americans in Benghazi, Libya, U.S. officials insist they need enough troops to protect their diplomats, and the legal authority to target those who might come after them, a senior U.S. official said.