President Obama has pledged to start withdrawing US troops from Afghanistan next month, with a goal of pulling all combat troops out by the end of 2014. He hasnt yet decided how many troops to pull this year. But since the killing of Osama bin Laden, domestic pressures have grown for him to front-load the drawdown. If the point was to squash Al-Qaeda, people ask, why are we keeping thousands of troops in Afghanistan? Having just returned from two weeks in Afghanistan and Pakistan, where I talked to US, Afghan and Pakistani leaders, Ive come to a different conclusion: The drawdown should proceed slowly, at least for this year. The situation in Afghanistan is far more perilous than is widely understood, but for reasons Obama has failed to explain clearly. As we saw last week - when militants made an unprecedented attack from Afghanistan into Pakistan - a failed Afghan state could become a haven for Pakistani militants who seek to destroy the Pakistani state. I sympathise with those who ask what is to be achieved by staying longer in Afghanistan. As the Afghan fighting season begins, and Taliban suicide bombs explode, its hard to see signs of progress, especially in a country whose government is rampantly corrupt. So let me lay out the US militarys strategy as it was explained to me, with caveats based on my own observations, along with a key reason for a slow withdrawal that Obama has never stated. First, lets revisit why we are in Afghanistan. As Gen David Petraeus put it to me in an interview in Kabul, were there so neither Al-Qaeda nor other groups can use Afghan soil to carry out transnational extremist attacks on our homeland. The only way (to prevent that) is to help our Afghan partners secure and govern their country ... to ensure that transnational extremists cant set up shop again, and prevent groups from taking control of the country. Can we stabilise Kabul sufficiently by 2014, the target date set by NATO for withdrawal of combat troops? Difficult, but not impossible. The military hopes to consolidate real security gains in the southern provinces of Helmand and Kandahar. This is the Taliban heartland, where US special forces and Army battalions are working with Afghan army and police. The goal is to thin out US forces very slowly in Helmand and Kandahar, after the current fighting season ends, as Afghan security forces become more capable. That capacity is being bolstered by additional training in literacy and in leadership skills. Meantime, US special forces carry out night raids targeting Taliban leaders. The US military is also aiding the creation of local village police forces, under control of district police chiefs, as a force multiplier. It is one thing to have Afghan forces from outside a district. It is very different when villagers themselves are working to defend their homes and valleys, Petraeus says. US advisers would remain in the south for now even as our forces thinned out. Certainly there will be a continued ISAF (international forces) presence, Petraeus says, though it will be reduced over time as Afghan forces and local government officials take on more security responsibilities. As US forces build up the south, they will shift some troops to the troubled eastern provinces that also border Pakistan. Now come the caveats. Illiteracy and attrition still pose serious problems to Afghan security forces; the police are notoriously corrupt. So, of course, is the central Afghan government of Hamid Karzai, although progress has been made in securing more competent police chiefs and governors at district levels. Moreover, this summers fighting season will be tough, as the Taliban focus on assassinating top officials. Despite US and Afghan efforts to encourage low-level Taliban to drop their guns and reintegrate into society, not many have taken up that option so far. Yet despite these caveats, and more, I believe it is essential to limit the troop drawdown this year. Heres why. Security progress so far is real but not consolidated. Afghan forces could collapse if left on their own too soon. But paired with US troops longer they might gain the confidence to fight on their own. My trip left me convinced that the much vaunted effort to start talks with the Taliban as an alternative to fighting is a real long shot. Theres little sign the Taliban are interested, nor do Western interlocutors know who can speak for the Taliban leader, Mullah Omar. A sharp pullback now could tell the Taliban there was no need to talk. But the biggest and most urgent reason to keep the drawdown limited, at least for this year, lies in Pakistan. Pakistani cooperation with the US military and CIA has been cut since the bin Laden raid - at a time when militants pose an existential threat to the country. A US rush to the Afghan exits would greatly embolden those militants. Better to withdraw slowly, work to build Afghan security forces, and mend ties with Pakistans army. Neither Obama nor Petraeus will say this out loud, but a prime reason we must stabilise Afghanistan is to prevent militants from harming Pakistan. Thats reason enough to give the current strategy more time. McClatchy