The enemy in Iraq is having a manpower shortage. This is noted by the reduction in the number of attacks on American troops, and the smaller groups of attackers involved in things like ambushes. This is one of the reasons for the new American policy of fighting it out with ambushes rather than hitting the accelerator. Because of money and recruiting problems, most ambushes in Iraq are conducted by a very small number of attackers. Unlike Vietnam, where the communists might deploy a hundred or so gunmen for an ambush, in Iraq ambush teams are most frequently 5-10 men. This is partially due to the different environment (not much jungle in Iraq) and partially to organizational ones (the enemy was a lot better organized and lot better trained in Vietnam). But mostly, it's a matter of money.
Most of the violence in Iraq is driven by organizations that pay people to participate. These groups have suffered increasing casualties over the last year, and that has resulted in fewer wealthy Sunni Arabs putting up the cash to pay the fighters. The increasing casualties caused by American and Iraqi forces has also discouraged Sunni Arabs from taking the money offered. The work has become too dangerous. The word gets around fast when Abdul and his four buddies all get killed when they tried to ambush some Iraqi police (and got spotted by an American UAV, and hit with a counterattack by Iraqi and American forces). Another problem for the recruiters has been increased use of night vision surveillance cameras by the Americans, and more UAVs available to fly night missions. The larger UAVs carry missiles. This means a few guys setting up a roadside bomb will suddenly disappear in a loud explosion. Another danger for the bomb planting teams are American snipers equipped with 12.7mm (.50 caliber) sniper rifles, and night vision scopes. Intelligence software can often predict which road the next bombs will be planted on, enabling the sniper teams to be deployed in such a way that they often catch these teams in the act, and often kill all three or four men involved. Actually, it's even better if one or two get away. They proceed to tell horrifying stories of their buddies "blowing up" when h it. That's a normal effect when someone is hit by a .50 caliber bullet. Those scary stories have a tendency to travel fast and far, discouraging others from hiring on to plant bombs. The recruiters have responded, as they always have, by raising the rates, But this means fewer teams can be sent out. With the money guys getting discouraged as well, there is less cash, higher fees for the gunmen and bomb planters and, as a result, far fewer attacks against Americans, and even Iraqi police and troops (who, the terrorists know, usually operate with American backup forces.)