Ronald E. Neumann was U.S. ambassador to Afghanistan from July 2005 to April 2007. He is the author of “The Other War: Winning and Losing in Afghanistan.”

As President Hamid Karzai meets with President Obama on Friday, their discussion is sure to include the number of U.S. troops in Afghanistan after 2014. There are many options, but decisions should depend on our objectives — which badly need to be clarified for Americans, Afghans and the world community. The media are full of numbers — 3,000 to 20,000 — that are said to have various levels of risk. But risk of what? The lack of clarity about the United States’ purpose in Afghanistan has confused Americans and Afghans alike. The latter, preoccupied not with transition next year but with survival in 2015 and beyond, resort to hedging strategies of tightening tribal and militia ties that undercut state development and Afghan military cohesion.

Obama’s announced goal of defeating al-Qaeda has never been well linked to our strategy of rolling back the Taliban insurgency and stabilizing Afghanistan — the efforts that preoccupy our troops and money. They are connected, but not in ways clearly explained. Al-Qaeda is a regenerative movement. Afghanistan lacks a reasonably firm state that can manage the large war that will continue after our combat forces are withdrawn. Without some continuing U.S. support, civil war and fragmentation are likely to engulf Afghanistan, destabilizing Central Asia as neighboring powers are drawn to safeguard their own interests and providing a breeding ground for extremism.

Acknowledging the cohesion between what we have been doing and defeating al-Qaeda is fundamental to clear decisions. Do the goals still hold, or are we changing them? The different troop numbers are not about “risk” but about different policies.

A presence of 3,000 to 6,000 troops is a counterterrorist policy that gives up on serious support for the Afghan military and focuses on killing our enemies. It offers nothing to Afghans except endless killing and, hence, will face increasing Afghan rejection. Further, since our forces will need local allies for intelligence and logistical support, such a tiny presence is likely to further empower the very warlords who have done so much to foment corruption. If the Afghan state collapses without our support, our presence will be unsustainable. In sum, a counterterrorist strategy is superficially attractive but a bankrupt strategic choice.

Stabilizing Afghanistan, the policy we have actually pursued, will not require the massive troop commitment in place today but will require three missions besides counterterrorism. One is training. Traveling throughout southern and eastern Afghanistan last year, I saw that Afghan forces have made significant progress. But much will remain to be accomplished in higher staff and institutional development if these forces are to stand on their own in a long war. How many trainers are needed can be debated, but the function should not be ignored.

Another mission is to build logistical, air and medical evacuation services (“enablers”). I was told during a visit to Afghanistan in 2010 that we could not complete required enabler training and equipment delivery before 2016 because we delayed starting the program in order to devote our efforts to rapidly forming infantry units. U.S. legal wranglings and contractual problems have caused further delays. Yet without these capabilities, Afghan forces will be confined mostly to garrisons, ceding the countryside to others, including insurgents. We need to fill the gap for a few years to succeed and because we share responsibility for the situation. Enablers can be steadily ramped down as Afghan forces develop, but they should not be abruptly cut off because we have failed to meet an earlier, never-achievable deadline.

Third, a small number of advisers will be needed with Afghan units. In addition to moving Afghan forces from quantity to quality, they will provide essential links to air and logistics. For example, U.S. pilots will not bomb targets unless Western advisers direct the strikes and evaluate the dangers on the ground. These requirements will diminish as Afghan forces and enablers mature.

Responsible decisions on force levels beyond 2014 need public clarity about which missions the forces are to accomplish, for what purposes and with public acknowledgment of risks to mission success. Deciding numbers without such clarity is a recipe for failure, because everyone from the Karzai government to our NATO allies will make different assumptions about our policy and work at cross purposes based on those assumptions.

Deciding on small numbers and claiming that the U.S. forces can do multiple training and counterterrorist jobs strains credulity. Changing to a mostly counterterrorist mission, in addition to its practical problems, would undercut our agreements to support Afghanistan beyond 2014 as well as the commitments our diplomacy has generated from NATO nations and other troop and financial contributors. In short, it would be a new policy in the last years before transition. I think it would fail, but if the Obama administration decides to change the policy, then it owes Americans, allies and Afghans an explanation about what we are really doing and how the potential for success of the new policy justifies the lives it is putting at risk.

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