SUKKUR, Pakistan  Men waded waist deep all week wedging stones with their bare hands into an embankment to hold back Pakistan’s surging floodwaters. It was a rudimentary and ultimately vain effort to save their town. On Thursday, the waters breached the levee, a demoralizing show of how fragile Pakistan’s infrastructure remains, and how overwhelming the task is to save it.

Even as Pakistani and international relief officials scrambled to save people and property, they despaired that the nation’s worst natural calamity had ruined just about every physical strand that knit this country together  roads, bridges, schools, health clinics, electricity and communications.

The destruction could set Pakistan back many years, if not decades, further weaken its feeble civilian administration and add to the burdens on its military. It seems certain to distract from American requests for Pakistan to battle Taliban insurgents, who threatened foreign aid workers delivering flood relief on Thursday. It is already disrupting vital supply lines to American forces in Afghanistan.

The flooding, which began with the arrival of the annual monsoons late last month, has by now affected about one-fifth of the country  nearly 62,000 square miles  or an area larger than England, according to the United Nations.