America has been internally peaceful for 150 years because ideas are allowed to flow freely with guaranteed rights to speech, religion and privacy. In other words, anyone can be whoever they want in America without hindrance. We are free to pursue life, liberty and happiness. As such, a democracy helps constrain ethnic divisions because people are free to exist however they want.
although it has always been the debate for every country to eventually move into a democratic society, it has been shown that even democratic societies have divisions. For instance The USA is supposed to be a very diverse nation, yet the representation of all Americans are usually middle aged "white" men.