With the Taliban on the march, its cities paralyzed by demonstrations and its president targeted four times for assassination, Pakistan is facing its most severe crisis since the 1999 coup that brought Gen. Pervez Musharraf to power. Over the past few months, surging Islamic extremism and widespread political unrest have erupted into violence, undermining the government's authority. Now, with elections on the horizon and the general's heavy-handed tactics aggravating tensions, Washington is being forced to reexamine one of its most critical and controversial alliances in the war on terror.