In the weeks and months following the 9/11 attacks, I was proud to lead an amazing, resilient, united people. When it comes to the unity of America, those days seem distant from our own. A malign force seems at work in our common life that turns every disagreement into an argument, and every argument into a clash of cultures. So much of our politics has become a naked appeal to anger, fear and resentment. That leaves us worried about our nation and our future together.