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.