A hundred and thirty thousand U.S. soldiers in Iraq were simply not enough to deal with a small fraction of that number of insurgents. It wasn't only because insurgencies, pace C. E. Callwell, arise from the soil itself, and thus have whole categories of advantages that a military force from the outside, alien to the culture, lacks. It was also because—as the large number of American troops near the Baghdad airport attested—the U.S. defense establishment was still organized for World War II and the Korean War, with too many chiefs at enormous rear bases, and too few Indians at the edges. In the weeks ahead the Marines at Fallujah would attempt to avoid large-scale bloodshed by seeking Iraqi surrogates to patrol the city. Such an expedient may provide a hint as to how the U.S. military will deal with Iraq as a whole.
I remember the Fallujah episode as a series of puzzling and inconsistent news reports, ending in an odd sort of stalemate. This story provides more context.
The Marines Kaplan describes are classic warriors -- samurai -- violently devout, righteous, often compassionate, aggressive, lusting for battle and glory, courageous and prepared to die in battle. They seem to resemble their opponents, save that the Marines seem far more concerned about civilian casualties. I suspect for the insurgents there is no such thing as a "non-combatant". Women and children are combatants too; and those who are not combatants do not merit life.
The Marines entered Fallujah thinking there were a minority of enemies in a civilian population that wanted them out. They discovered that "minority" was pretty large -- too large to kill. They found every mosque was a military facility. They had far too few men to pacify a city like Fallujah -- unless they were to kill tens of thousands of men, women, and children. Ultimately they were withdrawn. It was the least bad alternative given fundamentally mistaken assumptions.