Let me give a very brief summary of the book so far. First, up to the 1880s the U.S. was dominated politically by a "double consciousness". On the one hand, Anglo-Americans identified positively with their own dominant ethnicity. On the other hand, they were committed to values which led them to support open borders which undermined this ethnicity.
This changed in the later 1800s. The Protestant churches began to support the idea of helping to improve the social conditions of the working-classes. A progressive movement also emerged at this time, the dominant wing of which was in favour of patriotic immigration restriction. The worker's movement also sought to contain immigration in order to improve workers' conditions. For a period of time, this coalition was ascendant and was able to have new laws on immigration passed in the 1920s.
However, in this same period (1870s to 1920s) a cosmopolitan group of left-wing progressives also emerged who would eventually become dominant.
In the late 1860s a group was formed called the Free Religious Association. It promoted a new "religion of humanity" rather than Christian orthodoxy. It was cosmopolitan in the sense that it was accepting of non-Christian religions, although its leaders still identified positively with Anglo-Saxonism. However, it pushed the frontiers of thought closer to the idea of a future pluralistic, cosmopolitan nation.
Another step toward cosmopolitanism was the foundation of the Ethical Culture Society by Felix Adler. Adler was a leader within the Reform Judaism movement. This movement was anti-Zionist, teaching that the Jews were elected not to return to Israel but to spread the word of a universal faith that would unite the world's peoples in monotheism. For Adler, this meant that the Jews would universalise themselves out of existence as part of this process.
In other words, Jews were not to assimilate to an Anglo-Saxon ideal; rather, they were to help lead a movement in which all traditions would universalise and die out.
Adler's ideals cross-fertilised with those of radical Protestantism in the 1870s. A member of the Free Religious Association commented in the 1870s that:
...now between the most advanced Jews and the most progressive portion of Christendom there is scarcely a shade of difference in theological belief.Adler became president of the Free Religious Association in 1878; Kaufmann notes that,
This act inaugurated the Protestant-Jewish secular alliance that was to prove so potent an intellectual force in the twentieth century.
I want to pause here for a moment to make two brief points. First, it's important to recognise that prior to the 1870s, the Anglo elite had supported open borders. It's not likely that the older classical liberal/libertarian ideals would have supported a national existence in the longer run.
Second, this is all happening in the 1870s, a long time before the influence of Marxism can be said to really kick in. Although I don't doubt that the Frankfurt school had a major impact later in the 1900s, I don't think it can be seen as the origin of leftist cosmopolitanism.
Kaufmann next goes on to point out that the rise of progressivism, even though it was initially not a cosmopolitan movement, did open up more intellectual space for a "left progressivism" or "left-liberalism" to develop.
The next development was the emergence of the Settlement Movement in the 1890s. This movement placed well-educated young Anglo-Americans (mostly liberal Protestants) in the poorer parts of the cities in order to bring a higher culture to them. The Settlement Movements in Chicago and New York, in particular, developed toward a cosmopolitan view:
First, immigrants' culture was to be encouraged; it would be treated as a "gift" to the American amalgam. Second, the American nation would be implored to shed its Anglo-Saxon ethnic core and develop a culture of cosmopolitan humanism, a harbinger of impending global solidarity.
I'm going to stop here, even though some of the most extraordinary quotes are just coming up (you'll have to wait for these). Suffice it to say that by the early 1900s, there were Anglo-American intellectuals who believed it to be morally right for their own tradition to pass out of existence. Instead of thinking that it was through Anglo-Saxonism that the world would be brought to its liberal destiny, it came to be thought that it was through its decline that the new liberal world order would be brought into existence.