Among the things that emerge from every war, won or lost, are "lessons learned." Each war teaches the military on both sides strategic, operational, tactical and technical lessons that apply in future wars. Many of these lessons are useful. Some can be devastating. The old adage that "generals are always fighting the last war" derives from the failure to learn appropriate lessons or the failure to apply lessons properly. For example, the lessons learned from the First World War, applied to the Second, led to the Maginot Line. They also led to the blitzkrieg. "Lessons learned" cuts both ways.

Sometimes lessons must be learned in the middle of a war. During World War II, for example, the United States learned and applied lessons concerning the use of aircraft carriers, the proper employment of armor and the execution of amphibious operations. The Germans, when put on the defensive, did not rapidly learn the lessons of defensive warfare on a strategic level. The Allies won. The Germans lost. There were certainly other factors at work in that war, but the speed at which lessons are assimilated and applied is a critical factor in determining the outcomes of wars. It has been said that success in war is rooted in the element of surprise; it follows that overcoming surprise is the corollary of this principle.

Lessons are learned and applied most quickly at the tactical level. Squads, platoons and companies, which are most closely in contact with the enemy and have the most immediate thing at stake — their very lives — tend to learn and adapt the most quickly. One measure of morale is the speed at which troops in contact with the enemy learn and change. One measure of command flexibility is the extent to which these changes are incorporated into doctrine. In addition, a measure of command effectiveness is the speed at which the operational and strategic lessons are learned and implemented. It usually takes longer for generals to understand what they are doing than it does sergeants. But in the end, the sergeants cannot compensate for the generals, or the politicians.

In the Iraq war, both sides have experienced pleasant and unpleasant surprises. For instance, the Americans were pleasantly surprised when their worst-case scenario did not materialize: The Iraqi army did not attempt to make a stand in Baghdad, forcing the U.S. military into urban attritional warfare. And the Iraqi insurgents were pleasantly surprised at the length of time it took the Americans to realize that they were facing guerrilla warfare, and the resulting slowness with which the U.S. military responded to the attacks.

On the other hand, the Americans were surprised by the tenacity of the insurgency — both the guerrillas’ ability to absorb casualties and the diffusion of their command structure, which provided autonomy to small units yet at the same time gave the guerrillas the ability to surge attacks at politically sensitive points. And the insurgents had to have been surprised by the rapid tactical learning curve that took place on the U.S. side, imposing a high cost on guerrilla operations, as well as the political acumen that allowed the Americans and others to contain the insurgency to the Sunni regions.

In a strategic sense, the Iraqi insurgents had the simpler battle problem. Insurgency has fewer options. An insurgency must:

1. Maintain relations with a host population that permits for regrouping, recruitment and re-supply. While this can be coerced, the primary problem is political, in the need to align the insurgency with the interests of local leaders.

2. Deny intelligence to the enemy by using the general population to camouflage its operations — thus forcing the enemy to mount operations that simultaneously fail to make contact with insurgents and also alienate the general populace. Alternatively, if the enemy refuses to attack the population, this must be used to improve the insurgents’ security position.

3. Use the target-rich environment of enemy deployments and administrative centers to execute unpredictable attacks, thereby increasing the enemy’s insecurity and striking at his morale.

The guerrillas’ purpose is to engender a sense of psychological helplessness in their conventional enemy, with the goal of forcing that enemy to abandon the fight or else to engage in negotiations as a means of defense.

The guerrilla does not have to win militarily. His goal is not to lose. The essence of asymmetric warfare is not merely the different means used to fight the war, but the different interests in waging the war. In Vietnam, the fundamental difference between the two sides was this: The North Vietnamese had a transcendent interest in the outcome of the war — nothing mattered more than winning — whereas for the Americans, Vietnam was simply one interest among a range of interests; it was not of transcendent importance. Thus, the North Vietnamese could lose more forces without losing their psychological balance. The Americans, faced with much lower losses but a greater sense of helplessness and uncertainty, sought an exit from a war that the North Vietnamese had neither an interest nor a means of exiting.

Now, Vietnam was more of a conventional war than people think. The first principle of insurgency — drawing sustenance and cover from a local population — was a major factor before the intervention of main-line North Vietnamese units. After that, these units relied more on the Ho Chi Minh Trail than on the local populace for supplies, and on terrain and vegetation more than on the public for cover. It was at times less a guerrilla war than a conventional war waged on discontinuous fronts. Nevertheless, the principle of asymmetric interest still governed absolutely: The North Vietnamese were prepared to pay a higher price than the Americans in waging the war, since they had greater interests at stake.

The United States fought a counterinsurgency in Vietnam. It should have tried to reformulate the conflict as a conventional war. First, the Ho Chi Minh Trail was the strategic center of gravity of the war, and cutting that line would have been a conventional move. Second, operating in a counterinsurgency mode almost guaranteed defeat. Some have argued that the U.S. difficulty with counterinsurgency warfare is its unwillingness to be utterly ruthless. That is not a tenable explanation. Neither the Nazis nor the Soviets could be faulted with insufficient ruthlessness; nevertheless, the Yugoslav Partisan detachments drained the Nazis throughout their occupation, and the Afghan guerrillas did the same to the Soviets. Counterinsurgency warfare is strategically and tactically difficult.

The problem for occupying forces is that — unlike the insurgents, who merely must not lose — the counterinsurgents must win. And because of asymmetric interests, time is never on their side. The single most important strategic error the Americans made in Vietnam was in assuming that since they could not be defeated militarily, they might not win the war, but it was impossible that they could lose it. They failed to understand the principle of asymmetry: Unless the United States won the war in a reasonable period of time, continuing to wage the war would become irrational. Time is on the side of guerrillas who have a sustainable force.

But as in other guerrilla wars, the occupying power has shown itself to have less interest in occupying the country than the resistance has in resisting. It is not the absolute cost in casualties, but rather the perception of helplessness and frustration the insurgent creates, that eats away at both the occupying force and the public of the occupying country. By not losing — by demonstrating that he will survive intense counterinsurgency operations without his offensive capabilities being diminished — the insurgent forces the occupier to consider the war in the context of broader strategic interests.

The U.S. option in Vietnam was to impose a conventional model of warfare — much as the United States did in Korea, when it ignored the guerrillas and forced the war into a battle of conventional forces. It is even more difficult to impose a conventional war in Iraq than it might have been in Vietnam under an alternative American strategy. Here, attacking the insurgents’ line of supply is a tenuous strategy — not because the line does not exist, but because the dependency on it is less. The insurgents in Iraq operate at lower levels of intensity than did the Vietnamese. The ratio of supplies they need to bring into their battle box, relative to the supplies they can procure within their battle box, is low. They can live off the Sunni community for extended periods of time. They can survive — and therefore, in the classic formulation, win — even if lines of supply are cut.

The Sunni guerrillas in Iraq have all of the classic advantages that apply to insurgency, save one: There are indigenous forces in Iraq that are prepared to move against them and that can be effective. The Shiite and Kurdish forces are relatively well-trained (in the Iraqi context) and are highly motivated. They are not occupiers of Iraq, but co-inhabitants. Unlike the Americans, they are not going anywhere. They have as much stake in the outcome of the war and the future of their country as the guerrillas. That changes the equation radically.

All wars end either in the annihilation of the enemy force or in a negotiated settlement. World War II was a case of annihilation. Most other wars are negotiated. For the United States, Vietnam was a defeat under cover of negotiation. That is usually the case where insurgencies are waged: By the time the occupation force moves to negotiations, it is too late. Iraq has this difference, and it is massive: Other parties are present who are capable and motivated — parties other than the main adversaries.