It occasionally happens that a reader's e-mail is translated into dots and dashes and sent to me over Mr. Morse's wonderful electric telegraph. The sounder on my desk, opposite the inkwell and under the flypaper scroll, recently tapped out the following, from Jim McDonnell of Baton Rouge, Louisiana:

Could you please explain what's meant by the remark about U.S. forces being unable to fight battles of encirclement? Is it that there are too few of them in Afghanistan or are you saying that our forces are constitutionally incapable of that kind of operation? If the latter is the case, that would make a column all by itself.

It would, and it does. The problem is not numbers but tactical repertoire, or lack thereof. That deficiency, in turn, is a product -- like so much else -- of the American armed forces' failure to transition from the Second Generation to the Third.

Second Generation tactics, like those of the First Generation, are linear. In the attack, the object is to push a line forward, and in the defense it is to hold a line. As we saw in so many battles in and after World War I, the result is usually indecisive. One side or the other ends up holding the ground, but the loser retires in reasonably good order to fight again another day.

Usually, achieving a decision, which means taking the enemy unit permanently out of play, requires one of two things, or both in combination: ambush or encirclement. Modern, Third Generation tactics reflect an "ambush mentality," and also usually aim for encirclement. To that end, Third Generation tactics are sodomy tactics: the objective is to get in the other guy's rear.

On the defense, that is accomplished by inviting the enemy to attack, letting him penetrate, and then launching a counterattack designed to encircle him, not push him back out. This was the basis of the new, Third Generation German defensive tactics of 1917, and also the German Army's standard defense in World War II.

On the offense, the rule is not "close with and destroy" but "bypass and collapse." The goal is to penetrate deep into the enemy's rear, by stealth or by force (the Germans used a three, not two, element assault, and the largest element was the exploitation element), then roll up the enemy's forward units from the flank and rear while overrunning his artillery, headquarters and supply dumps. The same approach was used by the Panzer divisions on the operational level, leading to vast encirclements of hundreds of thousands of Soviet troops on the Eastern Front in 1941.

The U.S. military today knows little or nothing of this. It did attempt an operational encirclement of the Iraqi Republican Guard by 7th Corps in the First Gulf War, but that attempt failed because 7th Corps was too slow. On the tactical level, most American units have only one tactic: bump into the enemy and call for fire. The assumption is that America's vast firepower will then annihilate the opponent, but that seldom happens. Instead, he lives to fight again another day, like Osama and his al Qaeda at Tora Bora.

While the central problem here is conceptual – sheer ignorance of Third Generation tactics -- there is a physical aspect to it as well. On foot, American soldiers are loaded down with everything except the kitchen sink, and they will probably be required to carry that too as soon as it is digitized. To use tactics of encirclement, you need to be at least as mobile as your enemy and preferably more so. The kind of light infantry fighters we find ourselves up against in places such as Iraq and Afghanistan are just that, light. They can move much faster on their feet than can our overburdened infantry. The result is that they ambush us, then escape to do it again, over and over. Flip-flops in the alley beat boots on the ground.

As the students in my seminar at Quantico discovered early in the year, the decisive break, both in tactics and in organizational culture, is not between the Third and Fourth Generations but between the Second and Third. It is little short of criminal that the American military remains stuck in the Second Generation. The Third Generation was fully developed in the German Army by 1918, almost a century ago. It costs little or nothing to make the transition. To those who understand how the Pentagon works, that may be the crux of the problem.