For baby boomers, the minute it became apparent the United States would finally end its military involvement in Afghanistan, even as the Taliban rapidly took over the country, images from the shocking end of the Vietnam War — America’s longest before this one — came flooding back. They were certainly on the mind of the baby-boomer-in-chief when he spoke of the endgame in July: Now that the images and the reality of two bitterly divisive and costly wars ending in the triumph of America’s adversaries are converging so dramatically, those mulling the political impact of the debacle in Kabul should...