After the First World War, many Americans feared that the Communist Revolution in Russia would spread to the United States. Fear outweighed rational debate, leading to a clamp down on civil liberties, with thousands arrested without warrants. In response, a small group of individuals set up the American Civil Liberties Union (ACLU). In the years since then, the ACLU has evolved from a small organization to the nation’s principal defender of civil rights, playing a role in some of the most famous events in twentieth-century American history.