As Iraqi soldiers dug in three weeks ago to defend the northwestern city of Tel Afar, they were shocked to see waves of Islamist militants coming to battle in Iraqi military vehicles.

The line of Humvees, along with a number of powerful mortars, appeared to have been stolen by the insurgents only days earlier when they seized a sprawling base near the northern city of Mosul, said Ammar Tuma, a member of parliament’s Security and Defense Committee who received regular updates from the battle.

Since the group that calls itself Islamic State began its rapid takeover of large parts of Iraq on June 10, military officials and other witnesses have seen stolen government-issued weapons in battles and military parades in Iraq and Syria, where the group formerly known as Islamic State of Iraq and al-Sham, or ISIS, also controls stretches of land. […]