For tens of millions of Americans alive on Sept. 11, 2001, the images are indelible: Flames exploding from a tower of the World Trade Center against a brilliant blue sky. In the shock that followed those terror attacks, it was common to imagine that the world had changed forever. Just how, exactly, no one could know. Twenty years later, after so many other turns of history, the ragged withdrawal of U.S. troops from Afghanistan is a reminder of how the nation has struggled to answer the attacks.