The thesis statement from the passage's introductory paragraph is:
"However improbable it may seem, the arrival of around two million Yiddish-speaking Jews from Central and Eastern Europe in North America, between the middle of the nineteenth century and the middle of the twentieth, has turned out to be one of the most profoundly influential modern migrations."
This sentence encapsulates the main idea and sets the stage for the discussion about the impact of Yiddish-speaking Jews on American society and vice versa.