American intelligence agencies have told the Biden administration that if U.S. troops leave before a power-sharing settlement is reached between the Taliban and the Afghan government, the country could fall largely under the control of the Taliban within two or three years after the withdrawal of international forces. That could potentially open the door for Al Qaeda to rebuild its strength within the country, according to American officials.

The classified assessment, first prepared last year for the Trump administration but not previously disclosed, is the latest in a series of grim predictions of Afghanistan’s future that intelligence analysts have delivered throughout the two-decade-long war.

But the intelligence has landed in a changed political environment. While President Donald J. Trump pushed for a withdrawal of all forces even before the terms of the peace deal required it, Mr. Biden has been more cautious, saying Thursday that he does not view May 1 as a deadline he must meet, although he also said he “could not picture” troops being in the country next year.