Now at the ten-year mark, America's war in Afghanistan is still subject to competing interpretations of its progress, or lack of it. The data on casualties, violent incidents, number of Afghan army troops and much else can be—and are—parsed in different ways to reach different conclusions. When a war has gone on for this long, however, its sheer longevity has consequences, for the war itself and for how we think about it. Three thoughts in particular are relevant.

First, NATO's forces have worn out their welcome. They were welcome in many respects early in Operation Enduring Freedom and in some respects and by some Afghans even now. But the biggest motivation for insurgency in Afghanistan has for some time been opposition to foreign occupation. The longer the foreign troops have been on Afghan soil, the more grating has been their presence and the more Afghans there are who take up arms against the foreigners, no matter how many of their predecessors have been killed off.

Second, the amount of time that has transpired ought to make us skeptical that there are more corners in this war yet to be turned and more lights to see at the end of tunnels. One sometimes hears that it was only recently that a winning strategy was implemented, or that sufficient resources were applied to the task. Such hopeful expressions wear thinner with each passing year.

Third, we need to resist the psychological tendencies that commonly accompany costly efforts, especially the treating of sunk costs as if they were an investment to be recouped. The longer a war goes on, the more marked are such tendencies. There are ample indications of such tendencies in discussion of the war in Afghanistan. One hears the hope, echoing Lincoln, that those who have died will not have died in vain. But the dead will never come back, no matter what happens henceforth in the war. The only calculation to make is how much can be accomplished with still more dying.