How and why did the u.s. return to isolationism after wwi?

Germany's unfettered submarine warfare against American ships during World War I provoked the U.S. into abandoning the neutrality it had upheld for so many years. The country's resultant participation in World War I against the Central Powers marked its first major departure from isolationist policy. When the war ended, however, the United States was quick to leave behind its European commitment. Regardless of President Woodrow Wilson's efforts, the Senate repudiated the Treaty of Versailles that ended the war, and the United States failed to become a member of the League of Nations.

Indeed, isolationism would persist for a few more decades. During the 1920s, American foreign affairs took a back seat. In addition, America tended to insulate itself in terms of trade. Tariffs were imposed on foreign goods to shield U.S. manufacturers.

America turned its back on Europe by restricting the number of immigrants permitted into the country. Until World War I, millions of people, mostly from Europe, had come to America to seek their fortune and perhaps flee poverty and persecution. Britons and Irishmen, Germans and Jews constituted the biggest groups. In 1921 the relatively liberal policy ended and quotas were introduced. By 1929 only 150,000 immigrants per year were allowed in.

During the 1920s and 1930s, the preponderance of Americans remained opposed to enmeshment in Europe's alliances and wars. Isolationism was solid in hinterland and small-town America in the Midwest and Great Plains states, and among Republicans. It claimed numerous sympathizers among Irish- and German-Americans. William Jennings Bryan of Nebraska, Robert M. La Follette of Wisconsin, and George W. Norris of Nebraska were among western agrarian progressives who argued fervently against involvement. Assuming an us-versus-them stance, they castigated various eastern, urban elites for their engagement in European affairs.

Because the universities would allow them to investigate their theories, even if they weren't catholic things.