The establishment of the Afghan military and police forces, which are said by officials to number more than 320,000 members as of late last year, has been held up as one of the signal accomplishments of the United States-led presence here. By many accounts, the forces have continued to fight effectively in a number of areas across the country, even with far less of the air support and logistical assistance that the United States had been providing in past years.

But the Afghans are taking casualties at an alarming rate. In the first four months of 2015, more than 1,800 soldiers and police officers were killed in action, and another 3,400 were wounded, according to a Western military official, who spoke on the condition of anonymity to discuss figures not being officially released by the Afghan government. Those casualties are more than 65 percent higher than the amount during the same period last year, the official said.

Now, the militia plan suggests diminished confidence in the Afghan Army and police forces — important national institutions in a country with few of them. Indeed, the stated intent in creating nationalized forces was to replace the patchwork of militias around the country with a unified, better-trained body that was more accountable to the government.

Even as Afghan forces launched offensives in insurgent strongholds across the south this year, the military was caught flat-footed by the gathering Taliban forces in the north, according to local accounts and some officials in Kabul, the capital. By April, the Afghan Army was losing ground to the Taliban in several northern provinces.

In mountainous Badakhshan, the Taliban broke through army checkpoints, taking prisoners and beheading a number of soldiers. Gruesome accounts of the violence shook public confidence. The Taliban advance spread, threatening more than a few remote mountain districts, and suggesting an ambitious strategy to carve out territory in the north, which in the 1990s was the heart of the anti-Taliban resistance.