For many Afghans who remember the Soviet withdrawal in 1989, the impending US drawdown raises the specter of a return to the civil war of the 1990s. If the US and NATO are to prevent Afghanistan from falling apart yet again, the country will need an army that is cohesive, multiethnic and professional. It should be relatively small.

In many undeveloped societies, the army is the steel frame of the national state, and the final barrier between order and chaos. Afghanistan is no different. After the Soviets left Afghanistan in 1989, the government in Kabul lasted several years. Once the military collapsed, the country descended rapidly into civil war.

If the Taliban surge back after the US withdrawal and make a run at Kabul, which they may very well do, it will not be Afghanistan’s weak and notoriously corrupt civilian ministries that will keep the government from falling. Nor will it be poorly trained and lightly equipped police in the districts. It will be up to the armed forces.

If the army falters—or, worse yet, fractures along ethnic lines—all will be lost. Much of the US and NATO effort in Afghanistan will have been for naught, and the remnants of al-Qaeda and its affiliates will have a failed state in which to regroup.

With the focus on local-level counterinsurgency since mid 2009, much attention has shifted to the police and other local security forces—which, so the story goes, know more about the areas they are operating in and are therefore better at winning the struggle for the population. The problem is that a resurgent Taliban could easily overwhelm or capture local police and village-based militias, without a strong army capable of coming to their aid.

Focusing on local-level counterinsurgency may be a fine approach for now. But, as Western troops leave, counterinsurgency will no longer be an important mission for the US or NATO—which will have too few troops remaining in Afghanistan to worry about district governments, police forces or the intricacies of tribal politics. The focus will shift to keeping the national government intact.

It is high time the US and NATO began pulling the bulk of their combat forces out of Afghanistan. Yet, they cannot afford to look like the USSR when it withdrew—under fire, with the country falling apart all around, the Afghan army barely able to function and many of its units defecting en masse to the insurgency. The Afghan army must remain unified, operational and well-supported in at least most of the country.

Earlier this year, there was much talk about increasing the size of the Afghan army beyond the current goal of 171,600. The bigger the army, the higher the risk of break-downs in discipline, abuses against the population, widespread corruption, or splits along ethnic lines—any one of which could be fatal.

The Afghan army’s most important mission is to maintain stability and put down insurgencies that threaten the existence of the state. Counterinsurgency and stability operations require highly trained and motivated professionals, not large numbers of troops.

What matters is the overall quality of the army—especially when it comes to professionalism and cohesion. The threat that the army could split or collapse is far greater than not having enough forces.

The Afghan army has improved substantially since the US and NATO began paying serious attention to the military in late 2009. Pay reforms boosted recruitment and reduced corruption, Western advisors improved administration in Kabul and tactical proficiency in the field, new training and educational academies increased professionalism in the officer corps and many Pashtuns joined the army.

Yet, the military still faces serious problems. Corruption continues to plague parts of the army, especially personnel and logistics. Though the percentage of Pashtuns in the army roughly parallels their share of Afghan society as a whole, they are greatly under-represented in the officer corps. Nearly all the Pashtuns who join the army are from the east and north where the Taliban is not so strong; there are few recruits from the south. Fixing these problems is more important than increasing the size of the overall force.

President Karzai, who wants to maintain a large army but is worried his government will not be able to pay for it when Western forces withdraw, has called for a return to conscription. But if history is any guide, conscription in Afghanistan would further divide the country, alienate the population and cause fissures in the ranks. The respect and popular support enjoyed by the army is one of the most important assets of the Kabul government. If that erodes, the country’s leadership will not have a leg to stand on.

A large military capable of offensive operations might actually be bad for the security of the region. Chances are Pakistan would use its well-honed tools of covert war-by-proxy to undermine a large Afghan army. The Pakistani military is extremely worried about being encircled – by a hostile and much larger Indian military on its eastern flank and an increasingly pro-India Afghan government to the west.

Moreover, Afghanistan does not need substantial offensive capability, as long as the US agrees to keep Pakistani ambitions in check. All it needs is a force capable of maintaining internal stability. Building a large army, giving Afghanistan high-end military technology, or building too many commando units would create unnecessary temptations for hawkish Afghan officers to engage in military adventurism.

The most the US and NATO need to avoid being dazzled by the numbers and develop a small, highly professional army that accepts the principle of civilian control over the military, is infused with a strong sense of national purpose, and is not quite large enough to dominate the country. An army like that could, over time, facilitate transition to a more stable democratic order.