President Obama has pledged to start withdrawing U.S. troops from Afghanistan next month, with a goal of pulling all combat troops out by the end of 2014.

He hasn’t 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-Qaida, people ask, why are we keeping thousands of troops in Afghanistan? After all, bin Laden was found in Pakistan. So was top al-Qaida military leader Ilyas Kashmiri, reportedly killed by a drone strike on Friday.

Having just returned from two weeks in Afghanistan and Pakistan, where I talked to U.S., Afghan and Pakistani leaders, I’ve come to a different conclusion: The drawdown should proceed slowly, at least for this year.

The situation in AfPak is far more perilous than is widely understood, but for reasons Obama has failed to explain clearly. The risk is not just that Afghanistan could implode, but that nuclear-armed Pakistan could do likewise.

As we saw last month — when militants made an unprecedented attack from Afghanistan into Pakistan — a failed Afghan state could become a haven for Pakistani jihadis who seek to destroy the weak Pakistani state and gain access to its nukes.

The U.S. strategy

I sympathize 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, it’s hard to see signs of progress, especially in a country whose government is rampantly corrupt.

So let me lay out the U.S. military’s 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, let’s revisit why we are in Afghanistan. As Gen. David Petraeus put it to me in an interview in Kabul, we’re there “so neither al-Qaida 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 can’t set up shop again, and prevent (such) groups from taking control of the country.”

Can we stabilize Afghanistan 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 U.S. special forces and Army battalions are working with Afghan army and police.

The goal is to thin out U.S. 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, U.S. special forces carry out night raids targeting Taliban leaders.

The U.S. 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.

Warning signs

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 summer’s fighting season will be tough, as the Taliban focus on assassinating top officials. Despite U.S. 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. Here’s why.

Security progress so far is real but not consolidated. Afghan forces could collapse if left on their own too soon. But paired with U.S. 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. There’s little sign the Taliban are interested, nor do Western interlocutors know who can speak for the Taliban leader, Mullah Mohammed Omar. A sharp pullback now could tell the Taliban there was no need to talk.

Pakistan still weak

But the biggest and most urgent reason to keep the drawdown limited, at least for this year, lies in Pakistan.

Pakistan’s weak civilian government is in disarray, and its military is infiltrated by Islamists. The Pakistani army is having deep trouble fighting its own jihadis. It insists it didn’t know of bin Laden’s presence, which, if true, indicates deep failures in its intelligence agencies.

Pakistani cooperation with the U.S. military and CIA has been cut since the bin Laden raid — at a time when militants pose an existential threat to the country. A U.S. rush to the Afghan exits would greatly embolden those militants. It might also frighten Pakistan’s army into risky compromises.

The stakes are much too high to gamble on a policy that would not only accelerate Afghan collapse but would also risk a Pakistani collapse. Better to withdraw slowly, work to build Afghan security forces, and mend ties with Pakistan’s army.

Neither Obama nor Petraeus will say this out loud, but a prime reason we must stabilize Afghanistan is to prevent militants from grabbing Pakistan. That’s reason enough to give the current strategy more time.