In Guerrilla Evolution, Gary Brecher explains why attacks on US troops are down:
Killing US troops was the insurgents’ Plan A: ‘If we put enough bloody GIs’ bodies on US TV, the cowardly Yankees will run away!’ It was a reasonable idea, considering we pulled out of Somalia after losing only 18 men. But what the insurgents didn’t realize was that Americans had toughened up after 9/11. Casualties didn’t faze us like they used to. By election time the Iraqi insurgents had killed 1100 GIs, but Bush still won.Time for Plan B. Plan B is classic guerrilla doctrine: ‘the long war,’ where you attack the invaders’ local allies, not the foreign troops themselves. The idea is, if you wipe out Iraqi collaborators, the US is just a blind giant. He’ll stick around for a while, stumble over the countryside wrecking stuff, but sooner or later he’ll get sick of stubbing his toes and go home.
So the insurgents are ignoring the hunkered-down, heavily fortified American bases and hitting the key, soft targets: the Iraqi police.