"This is a very resilient enemy, it's an adaptive enemy and I don't think for a minute that the Taliban or their kind are going to kind of fade away into the dust here in the next year or two. That's not going to happen," Milley said.
Milley said the war's momentum has shifted in favor of the Afghan government, even though its army and police are taking heavy casualties 50 to 100 killed per week.
It is an "open question" whether the Afghans can sustain that loss rate, he said, but he also predicted that the casualties would not "break" the Afghans' will.
"These guys are absolutely determined to fight for their country, and they're doing a good job at it. And, yes, they are suffering," he said. "Is it sustainable or unsustainble? I think that's an open question."
He said the Afghan troops are resilient "aware that if they fail in their fight they'll live under Taliban rule again."