President Biden seems to have set a trap for himself—and for Americans, allied personnel and Afghans seeking to leave Afghanistan. Those civilians were stranded after Mr. Biden withdrew U.S. troops only to be surprised by the Taliban’s quick takeover. Mr. Biden promised to evacuate them by Aug. 31, and the Taliban said they’d hold him to that deadline. On Monday the administration signaled that it intends to abide by it. It won’t be enough time.