As the war in Afghanistan brings an end to the forever wars, experts worry about an experience similar to what the military faced after the Vietnam War: an exodus of exhausted troops, an increase in substance abuse and racism, and leaders who may be so focused on beefing up the defense budget that they don’t pay attention to the task of new warfare, even as China threatens Taiwan, Russia threatens Ukraine, and the diminished resources and landscapes caused by climate change send military planners scurrying to keep up.