HERAT, Afghanistan — One of the most powerful mujahedeen commanders in Afghanistan, Ismail Khan, is calling on his followers to reorganize and defend the country against the Taliban as Western militaries withdraw, in a public demonstration of faltering confidence in the national government and the Western-built Afghan National Army.

Mr. Khan is one of the strongest of a group of warlords who defined the country’s recent history in battling the Soviets, the Taliban and one another, and who then were brought into President Hamid Karzai’s cabinet as a symbol of unity. Now, in announcing that he is remobilizing his forces, Mr. Khan has rankled Afghan officials and stoked fears that other regional and factional leaders will follow suit and rearm, weakening support for the government and increasing the likelihood of civil war.

This month, Mr. Khan rallied thousands of his supporters in the desert outside Herat, the cultured western provincial capital and the center of his power base, urging them to coordinate and reactivate their networks. And he has begun enlisting new recruits and organizing district command structures.

“We are responsible for maintaining security in our country and not letting Afghanistan be destroyed again,” Mr. Khan, the minister of energy and water, said at a news conference over the weekend at his office in Kabul. But after facing criticism, he took care not to frame his action as defying the government: “There are parts of the country where the government forces cannot operate, and in such areas the locals should step forward, take arms and defend the country.”