As the government of Pakistan totters, we must face a fact: the United States simply could not stand by as a nuclear-armed Pakistan descended into the abyss. Nor would it be strategically prudent to withdraw our forces from an improving situation in Iraq to cope with a deteriorating one in Pakistan. We need to think — now — about our feasible military options in Pakistan, should it really come to that.

We do not intend to be fear mongers. Pakistan’s officer corps and ruling elites remain largely moderate and more interested in building a strong, modern state than in exporting terrorism or nuclear weapons to the highest bidder. But then again, Americans felt similarly about the shah’s regime in Iran until it was too late.

Moreover, Pakistan’s intelligence services contain enough sympathizers and supporters of the Afghan Taliban, and enough nationalists bent on seizing the disputed province of Kashmir from India, that there are grounds for real worries.

The most likely possible dangers are these: a complete collapse of Pakistani government rule that allows an extreme Islamist movement to fill the vacuum; a total loss of federal control over outlying provinces, which splinter along ethnic and tribal lines; or a struggle within the Pakistani military in which the minority sympathetic to the Taliban and Al Qaeda try to establish Pakistan as a state sponsor of terrorism.

All possible military initiatives to avoid those possibilities are daunting. With 160 million people, Pakistan is more than five times the size of Iraq. It would take a long time to move large numbers of American forces halfway across the world. And unless we had precise information about the location of all of Pakistan’s nuclear weapons and materials, we could not rely on bombing or using Special Forces to destroy them.

The task of stabilizing a collapsed Pakistan is beyond the means of the United States and its allies. Rule-of-thumb estimates suggest that a force of more than a million troops would be required for a country of this size. Thus, if we have any hope of success, we would have to act before a complete government collapse, and we would need the cooperation of moderate Pakistani forces.

One possible plan would be a Special Forces operation with the limited goal of preventing Pakistan’s nuclear materials and warheads from getting into the wrong hands. Given the degree to which Pakistani nationalists cherish these assets, it is unlikely the United States would get permission to destroy them. Somehow, American forces would have to team with Pakistanis to secure critical sites and possibly to move the material to a safer place.

For the United States, the safest bet would be shipping the material to someplace like New Mexico; but even pro-American Pakistanis would be unlikely to cooperate. More likely, we would have to settle for establishing a remote redoubt within Pakistan, with the nuclear technology guarded by elite Pakistani forces backed up (and watched over) by crack international troops. It is realistic to think that such a mission might be undertaken within days of a decision to act. The price for rapid action and secrecy, however, would probably be a very small international coalition.

A second, broader option would involve supporting the core of the Pakistani armed forces as they sought to hold the country together in the face of an ineffective government, seceding border regions and Al Qaeda and Taliban assassination attempts against the leadership. This would require a sizable combat force — not only from the United States, but ideally also other Western powers and moderate Muslim nations.

Even if we were not so committed in Iraq and Afghanistan, Western powers would need months to get the troops there. Fortunately, given the longstanding effectiveness of Pakistan’s security forces, any process of state decline probably would be gradual, giving us the time to act.

So, if we got a large number of troops into the country, what would they do? The most likely directive would be to help Pakistan’s military and security forces hold the country’s center — primarily the region around the capital, Islamabad, and the populous areas like Punjab Province to its south.

We would also have to be wary of internecine warfare within the Pakistani security forces. Pro-American moderates could well win a fight against extremist sympathizers on their own. But they might need help if splinter forces or radical Islamists took control of parts of the country containing crucial nuclear materials. The task of retaking any such regions and reclaiming custody of any nuclear weapons would be a priority for our troops.

If a holding operation in the nation’s center was successful, we would probably then seek to establish order in the parts of Pakistan where extremists operate. Beyond propping up the state, this would benefit American efforts in Afghanistan by depriving terrorists of the sanctuaries they have long enjoyed in Pakistan’s tribal and frontier regions.

The great paradox of the post-cold war world is that we are both safer, day to day, and in greater peril than before. There was a time when volatility in places like Pakistan was mostly a humanitarian worry; today it is as much a threat to our basic sec...