KABUL, Afghanistan (AP) — Mark Esper sought a firsthand assessment Sunday of the U.S. military's future role in America's longest war as he made his initial visit to Afghanistan as Pentagon chief. Stalled peace talks with the Taliban and unrelenting attacks by the insurgent group and Islamic State militants have complicated the Trump administration's pledge to withdraw more than 5,000 American troops.

Esper told reporters traveling with him that he believes the U.S. can reduce its force in Afghanistan to 8,600 without hurting the counterterrorism fight against al-Qaida and the Islamic State gr...