Bone-tired like everyone else in Kabul, Taliban fighters spent the last moments of the 20-year Afghanistan war watching the night skies for the flares that would signal the United States was gone. From afar, U.S. generals watched video screens with the same anticipation.

Relief washed over the war's winners and the losers when the final U.S. plane took off.

For those in between and left behind — possibly a majority of the allied Afghans who sought U.S. clearance to escape — fear spread about what comes next, given the Taliban's history of ruthlessness and repression of women. And for thousands...