When the United States gave more than 3,000 armored Humvees to Iraqi security forces over the past 12 years, U.S. officials could not have imagined that the humble utility vehicles would become a decisive weapon in the hands of Washington’s enemies from the Islamic State.

But that is exactly what has happened. Humvees were some of the 30 vehicles converted into mobile suicide bombs that the Islamic State used to blast through Iraqi security forces’ defenses during its three-day conquest of Ramadi in mid-May. The militants also used an armored bulldozer and at least one U.S.-made M113 armored personnel carrier. There’s a simple reason the militants are using Humvees and other armored vehicles as rolling bombs: Their protective armored plating prevents defenders from killing the trucks’ drivers before the militants can detonate their loads, while the vehicles’ capacity to carry enormous amounts of weight means the Islamic State can sometimes pack in a ton of explosives. Some of the bombs used inRamadi contained the explosive force of the deadly Oklahoma City bombing in 1995 that devastated a federal office building and killed 168 people.

The attack on Ramadi was the latest assault in which the Islamic State used armored Humvees as shock weapons to breach security force perimeters, scare beleaguered Iraqi troops into fleeing their positions, and become the centerpieces of flashy videos the group released through social media to its supporters around the globe. In addition to Ramadi, they were also used in attacks elsewhere in Anbar province in the ongoing battle for the Baiji oil refinery, as well as in Iraqi Kurdistan and Syria, according to Al Sweetser, chief of operational analysis and assessments at the Joint Improvised Explosive Device Defeat Organization (JIEDDO), which the Pentagon established in 2006 to develop ways of protecting American troops from what had emerged as the insurgents’ weapon of choice during the Iraq War. […]