For the insurgents, IEDs are a cheap and effective way to use what they have a lot of explosives left over from years of warfare with the Soviets and the Iranians.

An IED can always just be one component of a full-scale ambush, so you have to be extra cautious near standard ambush sites: alleys, tunnels, valleys, going under an overpass on the road. But it gets kind of tricky when someone’s looking to be destructive.

The primary component of IED deployment is camouflage. The rule is that they’re going to look like whatever there is a lot of in your area. For example, some of these cities in Iraq have streets that are littered with plastic bags and cans, so those are exactly the types of items that are used to make an explosive device. You’ve got to be super-vigilant in noting your surroundings. You head into a normally populated area at a busy time of day and there’s no one around? That’s a pretty clear sign that something is not right.

Also, if you come into an area and there are casualties lying around that have been there through a previous action, you shouldn’t be going up to those right away without first investigating the possibility that they’re booby trapped or mined. See, the Iraqi government is supposed to be taking care of these places, so American troops will be in one area, and then they leave for a while. It’s during that time away that all sorts of things can happen.

In Iraq, they’ve got a lot of artillery shells, which are really easy to set up. They already contain a detonator, and the explosive is already optimized to do a particular type of damage: anti-personnel or a high explosive. With the charges already optimized for certain types of damage, you don’t have to shape them or add other components. Of course the enemy also has raw explosives, which take a little more expertise but they can make a bigger IED.

There’s a variety of ways to detonate things: pressure-release switches or trip-cords; remote detonation using a blasting cap, or even radio waves. Wire is probably the best way to go if there’s an enemy who’s able to watch what’s going on. The downside is that it’s detectable. Wire-type IEDs are usually deployed in places where the enemy knows you’re going to be patrolling. Up around the Syrian border, where the Marines and the Army are sort of weeding out the insurgent population from time to time, the enemy knows their path and what direction they’re most likely coming from. That has caused a new wave of IEDs, where the enemy cuts a piece of concrete or pavement out of the road, puts the IED under it, paves back over it, and uses a remote device to detonate.

The primary defense against IEDs is to use unexpected routes. Sometimes you have to use certain paths, and you better be well-armored and have some explosive ordinance disposal people in tow. But if you can, find another path that allows you to set up over-watch positions before you send in the main convoy. That will defeat any sort of ambush that’s set up.