Eight years ago, a small number of U.S. personnel, working in tandem with local Afghan leaders, entered Afghanistan with a defined aim: to punish al-Qaida and overthrow the Taliban regime that harbored them. Over the past year, that mission has morphed into the much broader objective of rebuilding the Afghan state and protecting Afghan villages. Most recently, America’s top commander in Afghanistan, Gen. Stanley McChrystal, said a new strategy must be forged to “earn the support of the [Afghan] people … regardless of how many militants are killed or captured.”

Such an undertaking, amounting to a large-scale social-engineering project, is unwarranted. The cost in blood and treasure that we would have to incur — coming on top of what we have already paid — far outweighs any possible benefits, even accepting the most optimistic estimates for the likelihood of success.

The essential question now is not whether the war is winnable, but whether the mission is vital to U.S. national security interests.

From this perspective the current open-ended strategy fails. The United States and its allies must instead narrow their objectives. A long-term, large-scale presence is not necessary to disrupt al-Qaida. Indeed, that limited aim has largely been achieved, with the exception of capturing Osama bin Laden.

What we have seen over the past eight years is a classic case of mission creep. U.S. military operations today draw from the “clear-hold-build” model offered in the U.S. Army and Marine Corps Counterinsurgency Field Manual, a volume that didn’t even exist in 2001. It states, “Soldiers and Marines are expected to be nation builders as well as warriors rebuilding infrastructure and basic services.”

But for how long? Afghanistan has not made any progress toward being able to function without the assistance of the U.S. and its allies. Congress mandated that the Obama administration come up with a set of benchmarks to measure progress, but this list — supposedly 50-items long — has still not been presented publicly. And no wonder. These metrics, due by Sept. 24, will surely raise questions about whether such ambitious objectives can be achieved within costs acceptable to the American public.

The United States does not have the patience, cultural knowledge or legitimacy to transform what is a deeply divided, poverty stricken, tribal-based society into a self-sufficient, non-corrupt, and stable electoral democracy. And even if Americans did commit several hundred thousand troops and decades of armed nation-building, success would hardly be guaranteed, especially in a country notoriously suspicious of outsiders and largely devoid of central authority.

It is, of course, unreasonable for any administration to guarantee success in times of war. Planning will always fall short of our expectations, and no one can reliably predict the future. But we should be especially wary of nation-building. In a study of seven nation-building projects carried out since the end of World War II, the RAND corporation concluded that only two, Germany and Japan, could be characterized as unalloyed successes — a failure rate of 71 percent. The prospects in Afghanistan are worse. As the U.S. Senate Committee on Foreign Relations stated in an August 2009 report (.pdf), “Unlike Iraq, Afghanistan is not a reconstruction project — it is a construction project, starting almost from scratch in a country that will probably remain poverty-stricken no matter how much the U.S. and the international community accomplish in the coming years.”

Washington’s hope for nation-building and counterinsurgency, particularly in the context of Afghanistan, is not so much misguided as it is misplaced. Containing al-Qaida and disrupting its ability to carry out future terrorist attacks does not require a massive troop presence on the ground. Committing still more U.S. personnel to Afghanistan undermines the already weak authority of Afghan leaders, interferes with our ability to deal with other security challenges, and pulls us deeper into a bloody and protracted guerilla war with no end in sight.

As Robert Jervis, a professor of International Affairs at Columbia University, recently noted, “[President Obama] has devoted much more attention to how to wage the war than to whether we need to wage it.” It is becoming clear that going after al-Qaida neither requires a large-scale, long-term military presence in Afghanistan, nor does such a mission constitute a vital national security interest.