The generals publicly supported their intervention in Afghanistan, but in private they worried they were trapped. After 16 years, they feared they had produced “a recipe for endless war,” according to an American ambassador who met with them. But the generals, the ambassador said, “felt there was no alternative, no realistic alternative” than to continue fighting a doomed mission.

Those generals were Pakistani. Their meetings with the ambassador, Tom Simons, took place in 1996. Mr. Simons recounted the experience to the journalist Steve Coll in 2002, one year into an American mission in Afghanistan that has now also lasted 16, and which President Trump announced on Monday that his administration would extend.

There is a reason that Afghanistan’s conflict, then and now, so defies solutions.

Its combination of state collapse, civil conflict, ethnic disintegration and multisided intervention has locked it in a self-perpetuating cycle that may be simply beyond outside resolution.

“I’m not saying that state formation will never work in Afghanistan, but externally building, as we’re trying to do it, cannot work,” said Romain Malejacq, a political scientist at the Center for International Conflict Analysis and Management in the Netherlands.