Videos of and reporting on the United States’ withdrawal from Afghanistan — the chaos, the desperation, the fear — has been to younger generations a fresh horror, and has left older generations seeing double: through one lens, the fall of Kabul; through the other, the fall of Saigon. The world has once again played witness to the deadly whirlpool left as the United States withdraws its armed presence, leaving a brief vacuum before local power fills the void.