The existence of an anti-Semitic movement in the United States of America since the World War is a paradox that attracts attention at once. The most ancient and most pervasive form of intolerance is now at home in a nation founded by revolution and dedicated to the principles of freedom and tolerance. How can such a movement exist in such a nation? The apparent contradiction leads us at once into the many contradictions of the psychology of large groups of human beings, which both parallels and contradicts the simpler psychology of their constituent individuals. This is a leading question, to answer which we must go as deeply as we can into the mind of the group, into the relation of groups to the smaller groups of which they are composed and of those smaller groups to each other, into the genesis and implications of tolerance and intolerance. This theoretical study completed, we shall then have to verify the principles there worked out by application to the difficult and crucial problem of the present study. If a theory of group and sub-group can explain the existence and the development of anti-Semitism in America, it will have solved a problem of exceptional complexity and significance, one central to the whole field. This will involve a study of the mind of the American people, in brief outline, with its various movements of intolerance in their bearing on the present one. It will also necessitate a slight study of the various anti-Semitic examples, historic and contemporary, from which the American movement derives in part. It will conclude with a consideration of the future of the American people as a united group, taking into view the tendencies of the sub-groups within the bounds of their common nation, or over-group. Anti-Semitism is the modern form of the ancient prejudice against the Jew; it began in Germany in 1871, directly after the Franco-Prussian War, and bases its opposition to the Jews on the race theory. Anti-Judaism is, of course, much older, as old as the people against whom it was directed. In most ancient times, as represented by the Egyptian taskmasters and the Haman of the Book of Esther, it was like any other national hatred or prejudice. Later it took on a distinctly religious coloring, so that we find a Philo going to Rome to appeal for the Jewish colony in Alexandria or a Josephus writing a defense of his people against Apion. With the growth of Christianity into a persecuting body, anti-Judaism became strictly a religious matter, based on the New Testament story that the Jews were responsible for the death of Jesus. Medieval laws on the Jews were, then, often based on the principle of expiation, such as the yellow badge which distinguished the wearer when he left the compulsory shelter of the Ghetto. A different form of religious motivation was shown in the frequent accusations of desecrating the Host or of using the blood of a Christian child in preparing the unleavened bread of Passover, which appears in the Canterbury Tales and was revived as recently as 1911 in the notorious Beilis case at Kiev, Russia. Along with this went occasional mob outbreaks such as occur against the negroes in our Southern states, and still more rarely decrees of expulsion, which drove the entire Jewish population from England in 1294, from Spain in 1492, and from other countries at other times, for a longer or shorter period.