There are dark pages in American history in addition to the stories about slavery and the mistreatment of those called Indians by the invading settlers.
-- We've seen a repressive federal government deny basic civil liberties to dissenters -- in 1798 with the Alien and Sedition Acts and in 1919-20 during the Red Scare.
-- We've seen the rise of political movements based on the hatred of people who were different -- the Know Nothings in the 1850s, the Ku Klux Klan both during Reconstruction and in the 1920s, and the anti-immigration activists who got laws barring Chinese and Japanese immigration as well as the 1924 law aimed at Jews, Italians, and others from southern and eastern Europe.
-- We've seen powerful movements promoting an isolationist foreign policy -- notably the America First groups in the 1930s.
-- We've seen successful political demagogues, though mostly contained to localized power -- racist Senators like "Pitchfork Ben" Tillman and Theodore Bilbo, more recently Huey Long and George Wallace.
But never before have we had this perfect storm of a skilled demagogue capturing a major political party with proposals of ethnic and religious repression, rejection of basic rule of law principles, and a crazy passive-aggressive foreign policy that threatens everybody and promises to punish America's allies.