The guerilla warfare encountered in Afghanistan and Iraq has led the U.S. Army to modify it's training procedures considerably. This is a common army reaction to situations like this, in a tradition going back to World War II. Actually, current army training already includes a lot of "irregular warfare" type situations, based on Balkans peacekeeping experience from the 1990s. But now the training makes greater use of civilians (either GIs in civilian clothes or local civilians hired, or volunteering, for the occasion) and more capable irregular fighters. The troops running the OPFOR (opposing force) operation try to make their operations as realistic as possible. This not only provides good training for American troops, but also provides insights into how the enemy operates. Nothing like going out and playing the role of an Iraqi irregular or Taliban fighter and seeing what you can get away with against American troops. It's only training, so failure isn't fatal, and is often quite educational.