The move by Taliban-backed militants into the Buner district of northwestern Pakistan, closer than ever to Pakistan's capital of Islamabad, have prompted concerns both within the country and abroad that the nuclear-armed nation of 165 million is on the verge of inexorable collapse.

On Wednesday a local Taliban militia crossed from the Swat Valley — where a February cease-fire allowed the implementation of strict Islamic, or Shari'a, law — into the neighboring Buner district, which is just a few hours drive from Islamabad (65 miles, separated by a mountain range, as the crow flies). (Read "At Pakistan's Red Mosque, a Return of Islamic Militancy".)

Residents streaming from Buner, home to nearly a million people, told local newspapers that armed militants are...