KANDAHAR, Afghanistan—By most objective measures, President Donald Trump’s year-old strategy for ending the war in Afghanistan has produced few positive results.

Afghanistan’s beleaguered soldiers have failed to recapture significant new ground from the Taliban. Civilian deaths have hit historic highs. The Afghan military is struggling to build a reliable air force and expand the number of elite fighters. Efforts to cripple lucrative insurgent drug smuggling operations have fallen short of expectations. And U.S. intelligence...