The Oct. 13 attack near the border with Pakistan, like dozens of others this year, was carried out by a member of Afghanistan’s security forces. Abdul Wali had joined the National Directorate of Security, Afghanistan’s intelligence service, after six years as a border guard. He had no known ties to the Taliban or personal grievances. His motive may never be known.

Interviews with commanders and soldiers in Kandahar provided graphic details of several so-called insider attacks and illustrated how deeply they are dividing U.S. forces from the Afghan army and police units the Americans have promised to mentor, train and fight with for at least two more years. …

U.S. and Afghan units still fight and train closely together every day, but often there is wariness and suspicion.

There have been 41 insider attacks by Afghan soldiers and police this year, killing 35 Americans and 22 people from other countries with forces in Afghanistan. Cmdr. William Speaks, a Pentagon spokesman, said “significantly more” Afghans than foreigners have died in the attacks.