And while the number of improvised explosive devices, or IEDs, found and troops injured or killed have plummeted in Iraq, they spiked recently in Afghanistan, reflecting the escalating combat there.
Army Lt. Gen. Thomas Metz, who heads the military's counter-IED organization, said that in late spring, the number of IED incidents peaked at about 200 a month in Afghanistan. Those incidents, he said, resulted in some of the highest monthly coalition casualties due to roadside bombs in the past four years. The number of IED incidents dipped in July.
Metz, who was the No. 2 commander of U.S. forces in Iraq in 2004-05, said the enemy was more likely to use explosives triggered by cell phones or other similar radio waves. But now militants on both warfronts have moved to less sophisticated triggering mechanisms, such as command wires or pressure plates.
Some of the shift came as the U.S. developed better sensors and other technologies to detect and defeat the more sophisticated models.
But the low-tech bombs, he said, "create a harder problem for us. We are seeing that, more and more."
Metz said it is difficult for troops - rolling down the road at about 30 mph - "to have a device that can look into the ground and detect, at a very low false positive rate, a pressure plate that's under there." A "false positive" refers to an indication that signals a bomb is there when it's not.
Still, he said, it takes longer for militants to bury a pressure plate, making it more difficult for them to plant an IED without being seen. Although the rural, often mountainous terrain in Afghanistan can provide more hiding places.
Roadside bombs have long been the premier killer in the wars, that - on average in Iraq - claims one casualty for every eight bombs planted and found. In 2004, Metz said, the average was far worse, at about one casualty for each bombing incident.
Metz is in charge of the Joint Improvised Explosive Device Defeat Organization, which is spending about $4 billion a year to research, develop, and train troops in high-tech equipment to detect and destroy the homemade bombs.
In an hourlong briefing Wednesday, Metz said that part of the spike in bombs and casualties in Afghanistan is due to the increase in coalition forces and combat activity there in recent months.
Pointing to a graphic showing the Afghanistan bombings over the past six years, Metz noted that they are "very seasonal, but obviously going up."
While he would not provide exact numbers because they are classified, Metz said that about 20 percent of the roughly 200 roadside bomb incidents in Afghanistan in April resulted in casualties. In about half of the incidents the bombs were found and removed by troops, in about another 80 incidents the bombs went off but did no damage.
The numbers in Iraq are higher, but the decline in bombings there has also been more dramatic.
At the peak in Iraq, there were as many as 3,000 roadside bombs discovered or detonated in a month, and that total has been more than cut in half, he said.
Metz added that one of his primary worries is that militants around the world will begin to believe that IEDs are the weapons of choice, making them a bigger potential threat to U.S. homeland security. Right now, he said, there are about 300 roadside bomb incidents a month around the world - not counting Iraq or Afghanistan.