It’s not just generals who are always prepared to fight the last war. President Biden’s April 14 announcement that U.S. forces will leave Afghanistan before the 20th anniversary of 9/11 has a long and complicated backstory. Biden said his decision will allow America to put this violent and ambiguous past behind it, to retire the frameworks that conditioned its foreign policy for a generation, and to focus its energies on the competition with China.Perhaps so. The risk, however, is that Biden’s fixation on settling old scores has blinded him to contemporary realities, and has prevented him from answering the question...