By George Friedman

The war in Afghanistan has been under way for more than 10 years. It has not been the only war fought during this time; for seven of those years another, larger war was waged in Iraq, and smaller conflicts were under way in a number of other countries as well. But the Afghanistan War is still the longest large-scale, multi-divisional war fought in American history. An American soldier's killing of 16 Afghan civilians, including nine children, on March 11 represents only a moment in this long war, but it is an important moment.

In the course of the Iraq and Afghanistan wars, military strategists in the United States developed the concept of the long war. The theory was presented in many ways, but its core argument was this: The defeat of Taliban forces and the Iraqi resistance would take a long time, but success would not end the war because Islamist terrorism and its supporters would be a constantly shifting threat, both in the places and in the ways they would operate. Therefore, since it was essential to defeat terrorism, the United States was now engaging in a long war whose end was distant and course unknown.

Sometimes explicit but usually implicit in this argument was that other strategic issues faced by the United States should be set aside and that the long war ought to be the centerpiece of U.S. strategic policy until the threat of Islamist terrorism disappears or at least subsides. As a result, under this theory — which very much influences U.S. strategy — even if the war in Afghanistan ended, the war in the Islamic world would go on indefinitely. We need to consider the consequences of this strategy.

Staff Sgt. Robert Bales, who allegedly perpetrated the appalling slaughter in Afghanistan, was on his fourth tour of combat duty. He had served three tours in Iraq of nine, 15 and 12 months — he had been at war for three years. His tour in Afghanistan was going to be his fourth year. The wars he fought in differed from prior wars. Fallujah and Tora Bora were not Stalingrad. Still, the hardship, fear and threat of death are ever-present. The probability of dying may be lower, but it is there, it is real, and there are comrades you can name whom you saw die.

In Vietnam, only volunteers served more than a single one-year tour. For Americans in World War II, the war lasted a little more than three years, and only a handful of U.S. troops were in combat for that long. U.S. involvement in World War I lasted less than two years, and most U.S. soldiers were deployed for a year or less. In U.S. history, only the Civil and Revolutionary wars lasted as long as Bales had served.

Atrocities occur in all wars. This is an observation, not an excuse. And they become more likely the longer a soldier is in combat. War is brutal and it brutalizes the souls of warriors. Some resist the brutalization better than others, but no one can see death that often and not be changed. Just as important, the enemy is dehumanized. You cannot fight and fear him for years and not come to see him as someone alien to you. Even worse, when the enemy and the population are difficult to distinguish, as is the case in a counterinsurgency, the fear and rage extends to everyone. In Bales' case, it extended even to children.

It is no different for the Taliban save two things. First, they are fighting for their homeland and in their homeland. Americans fight for the homeland in the sense that they are fighting terrorism, but that fight becomes abstract after a while. For the Taliban it is a reality. Americans can go home and may become bitter at those who never shared the burden. The Taliban are at home, and their bitterness at those who did not share the burden outstrips the bitterness of the Americans. Second, it is a fact of war that Taliban atrocities are usually invisible to the Western media, but they are there, even if reporters are not. It could be said that the Taliban were brutalized by years of fighting before the Americans came, but in the end, the fact of brutalization is more important than the genesis.

It is important to remember that for the United States, the Afghanistan War is the first major war since the Civil War that did not involve a draft. Opposition to the draft during Vietnam gave rise to the volunteer army. One thing no one assumed after Vietnam was that the United States would attempt to fight a counterinsurgency on the mainland of Asia again, and therefore the conditions for reconstituting the draft were never considered.

When the war in Afghanistan began, there was no theory of the long war. It was assumed that the goal was the dislocation and destruction of al Qaeda, and grandiose notions of democratizing Afghanistan were not yet part of the policy. In Iraq, the assumption was that the defeat of Saddam Hussein's conventional forces would require neither significant cost nor time and that there would be no resistance to constructing a pro-American democracy there. It took time for the mission in Afghanistan to creep up to democratization, and it took a while to realize that not all Iraqis were cheering the American occupation.

But even while it became apparent that the United States was in a long war, neither the Bush nor the Obama administration ever grappled with the consequences of a force in which individuals could be in combat for four years and more. And we might include here the dangers for noncombatants and headquarters troops, who faced mortar and rocket fire at their desks. No one escaped the burden.

The result was a war that was seen on the home front as not requiring a massive effort but that required some volunteers to remain in combat for longer than many had in World War II. And while it was true that all of the soldiers had volunteered, the volunteers were no more ready than the government for the tempo of operations they would face. Additionally, they were not always free to leave. During the height of the war, some of those trying to leave service when their time was up were "stop-lossed." For them, it became less of a volunteer army than a captive army.

The doctrine of the long war fought by the present force fails to take into account whether the force can sustain the war. Former Defense Secretary Donald Rumsfeld argued that you fight with the army you have. What he did not address was that while you begin fighting with the army you have, as the United States did in World War II, you do not continue fighting with that army, but move to mobilize the country. But Rumsfeld did not realize how long the war in Afghanistan would last, and in particular, he did not anticipate the cost that two multi-divisional wars would have. It is noteworthy that Bales began with three tours in Iraq. The war in Iraq might be over, but its consequences for the force remain.

What Bales is alleged to have done is inexcusable. There have been many atrocities, both recorded and not, both outright and ambiguous, and conducted by both NATO and the Taliban. It is unrealistic to imagine a war of this length devoid of atrocities. But in a counterinsurgency, in which the goal is not simply the defeat of an enemy force but also persuading the population that turning against that force is the safest course, a massacre like this can have strategic consequences. The Taliban's psychological warfare operations will focus on the killings as they did with the February Koran-burning incident at a U.S. base. In the meantime, American psychological warfare efforts will focus on U.S. troops, both making sure they remain restrained and — after the Feb. 25 shooting of two U.S. officers in a Kabul ministry by an Afghan colleague — reassuring them that they must not be afraid of Afghans, since training Afghans is their mission.

The long war, without a major readjustment of the American force structure, creates unintended strategic consequences. One consequence is a force that contains large numbers of troops at the limits of their endurance. Their potential actions undermine the strategic purpose of the counterinsurgency: winning over the populace. That opens the door to increased influence for the Taliban and reduces the Taliban's inclination to negotiate as the U.S. position deteriorates. Put differently, troops are not numbers on a table of organization. They wear out.

There are four strategic assumptions of the long war underlying all of this. The first is that the fight against Islamist terrorism can be won and that ultimately it is more than just a threat that has to be accepted. The second is that large-scale operations like those in Iraq and Afghanistan help achieve that goal. Third, that the United States is able to wage a long war such as this without massive adjustments to its domestic life. Fourth, that this should continue to be the centerpiece of U.S. strategy indefinitely, regardless of other events in the world — in other words, that this is the single most important challenge facing the United States.

The invasion of Afghanistan was strategically justifiable as a means of disrupting al Qaeda and preventing follow-on attacks against the United States. The invasion of Iraq was based on a false assumption that the Iraqis would not resist occupation. As the wars went further, the military situation became more difficult while the goals expanded. The ultimate expansion was the idea that the United States was committed to an indefinitely long war, with available forces, and that this would involve occupying large and hostile countries.

I argued in my last book, "The Next Decade," that the danger of empire was that it threatened the republic. After the fall of the Soviet Union, the United States became the world's only superpower, combining military, economic and political might on a global basis. Whether it wanted this power or not, it had it. Within a decade of the Soviet Union's collapse, 9/11 happened. Whatever its initial intentions, the United States found itself in a war that has lasted more than 10 years. That war has strained American resources. It has also strained the fabric of American life.

The threat to the republic comes from multiple directions, from creating systems for national defense that undermine republican principles to overestimating military capability and committing the republic to a war whose end state is unclear and where the means are insufficient. War transforms countries, and the long war transforms domestic life and creates an unbalanced foreign policy. Most of all, it creates a professional class that fights wars that are considered limitless while the rest of society, though paying the bills, does not see the war as being part of everyday life. The alienation between citizen and soldier in a nation struggling to reconcile global power with republican institutions is historically dangerous.

This is made all the more dangerous because the force is reaching its limits. Resisting terrorism is important. Eliminating it is an illusion. To continue with the long war with the forces available puts in motion processes that threaten the republic without securing U.S. interests. Leaving aside the threat to the republic, a force at its limits and left to fight a war on the margins of national consciousness will not be effective.