Islamic State's most effective weapon? Captured Humvees converted to car bombs. Yahoo/Business Insider

ISIS militants with a captured Humvees. Yahoo/Reuters. 

ISIS is using the thousands of captured Iraqi Humvees as extremely effective car bombs. Yahoo/Business Insider. ISIS has captured more then 2/3rds of the heavy utility vehicles that the United States have provided to Iraq. In Mosul alone they captured 2300 Humvees. The vehicles are being used in the fight in Iraq and many have made their way to Syria as well. Humvees are extremely effective as suicide car bombers. They are very resistant to small arms fire and can carry up to a ton of explosives. They are also capable of running through barriers and exploding before the driver can be killed. ISIS has used waves of attacks with multiple car bombs to drive back Iraqi forces. In Ramadi, ISIS used 30 or more car bombs, some of which were comparable to the Oklahoma City Bombing in terms of scale, and then followed the bombings up with regular infantry attacks. 

My Comment:

When you really think about it, Humvees are about the perfect vehicle for suicide car bombing. They can carry a massive amount of explosives. They are also highly resistant to rifle and machine gun fire. Remember, these Humvees are the up-armored versions that have been designed to better survive IED explosions. Ironically enough, these IED resistant Humvees make perfect platforms for IED's. There is also an identification problem for the Iraqi forces. Since they use Humvees as well, they have to hesitate, which is the last thing you want to do when a rolling bomb is heading at you. They are also useful as a "point of the spear" attack that can clear the way for more conventional car bombings or even infantry assaults. 

There isn't all that much that Iraqi infantry can do against them either. Rifle fire will mostly be unable to penetrate the Humvees. Heavy machine guns might do a better job, but even a wounded driver can blow his vehicle up. Anti-armor rockets could work as well, but since Humvees move a lot faster then armor, it would be very hard to hit one. And even if you do hit it, you still might be in the blast radius when it blows up. 

Tanks and APC's might do a bit better then infantry alone. Tanks and APC's have enough range and firepower to destroy the car bombs. They would also have the ability the maneuver and evade the bombs as well. Still, a Humvee packed with a ton of explosives is one of only a few things that could destroy American made tanks like the M1A1 Abrams. And they would have the speed and maneuverability to make them very hard to hit compared to a heavy vehicle like a tank. 

There are other options, though none of them are good. Iraqi has tried to use barriers and tank traps to try and protect their forces from these car bombings. But ISIS has also mastered countering barriers. Not only can the Humvees often bash through these barriers, ISIS has a whole corps of modified and up-armored bulldozers to destroy the barriers and poke a hole in the barriers the car bombs can get through. There are also mines and IED's but those tend to cause problems of their own. And the Humvees are resistant to them... Here's a nice video of when it goes right though... Not sure if this was a Humvee or what, but it's a proof of concept though. 

The effectiveness of ISIS car bombing strategy is pretty obvious. The destructive power of the bombings was a major factor in the battle of Ramadi. Iraqi troops don't have an effective counter for the tactic and the effect they have is devastating. Not only do these attacks cause multiple casualties, they have a terrible effect on morale. Any attack that you can't defend yourself against it going to utterly destroy your morale. It really helps to explain why Iraqi forces folded in Ramadi, though it's far from the only reason. 

The best counter I can think of for stopping these attacks is by destroying the factories that make the car bombs, killing the bomb makers and destroying any explosive supply caches the enemy may have. Though the United States has had some success lately in destroying these factories, but much more needs to be done. The problem is that there just isn't enough intelligence to find where these bombs are being made. And Iraq and Syria are littered with explosives to use in such attacks. 

I haven't mentioned it yet, but the Humvees that ISIS have captured also work very well for more conventional purposes. They are good all around utility vehicles and are very useful as lightning raiders when loaded up with infantry. They offer better protection then the pick up trucks that ISIS had to use before they captured the Humvees, and are more then a match for anything except armor or troops with air support. Their only real limitation is that they are gas guzzlers, but we are talking about a region that pumps oil, so that's probably not much of a concern. 

Of course, none of this would be happening if ISIS hadn't captured Mosul and the 2,300 Humvees and other vehicles abandoned there by the Iraqi Army. The Army abandoned more vehicles at Ramadi as well, adding more fuel to the car bomb fire. I know that hindsight is 20/20 and foresight is blind, but this is a direct consequence of arming the Iraqi Army without knowing that they would fight. We are paying the price for that mistake now.