In his influential 1882 essay “What Is a Nation?” French philosopher Ernest Renan wrote about the bonds that hold nations together. He explained, “A heroic past, great men, glory [are the links between people] upon which one bases a national idea. . . . A nation is . . . a large-scale solidarity, constituted by the feeling of sacrifices that one has made in the past and of those that one is prepared to make in the future.” Others have stressed language, ethnicity, or even pseudo- scientific ideas about “race.” The migration of people between one nation and another is challenging long-held assumptions about who belongs.