No nation is more critical to U.S. foreign policy than nuclear-armed Pakistan. Wedged between India and Afghanistan, it is the second largest country in the Islamic world. But with militant Islamists now expanding their control over some of the country's most strategically sensitive areas, there is a growing fear that Washington's most stolid ally in South Asia--at least ostensibly--is unraveling, and perhaps is even on the verge of collapse. With a dozen or so private Islamist armies, a hundred or so nuclear weapons, and a dysfunctional government, Pakistan is considered one of the most...