Dear Mitt Romney,Congratulations on Florida. Now that you are again the front-runner, and your campaign focus is returning to President Obama, I’d like to call attention to a line you have used repeatedly: “This is a president who fundamentally believes that this next century is the post-American century.” I leave it to the president to describe what he believes, but as the author of the book “The Post-American World,” let me make sure you know what exactly you are...