In 1893-4 there were almost 800 German-language publications in the US. "With the decline of immigration and the consolidation trends after 1890, German-language publications declined to 613 in 1900 and 554 in 1910...*World War I accelerated an ongoing decline.* [my emphasis--DT] *Harvard Encyclopedia of American Ethnic Groups,* p. 420 (article "Germans")
The main reason German was a flourishing language in the US in the early twentieth century was simply that there were so many fairly recent German immigrants. As late as 1890 Germans represented 30.1 percent of the foreign-born in the US--that went down to 25.8 percent in 1900 and 17.1 percent in 1910 and then to 12.1 percent in 1920--that last figure being only in part a result of the War. With the decline of German immigration, the German language was likely to decline. This happened with other languages, too. "In 1923, when the Polish-American journals were at their height, at least 19 dailies, 67 weeklies and 18 monthlies were in circulation." *Harvard Encyclopedia of American Ethnic Groups,* p. 800 (article "Poles") After 1930, "the number of the Polish press titles...began to decline steadily."
https://books.google.com/books?id=Jyd_DAAAQBAJ&pg=PA202 Yet the US had never gone to war with Poland...
Again, I am not denying that the world wars encouraged German-American assimilation--but they only accelerated a trend that would have taken place anyway.
https://www.alternatehistory.com/fo...cuisine-in-the-us.415902/page-2#post-14738637
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In general, a non-English language in the US needs
new immigrants to avoid decline through assimilation. Latin America supplied the new immigrants for Spanish. Remember that even the restrictive legislation of the 1920's did not have any quota limits for the Western Hemisphere. (Ironically, those were first set by the 1965 law which many people mistakenly think led to the upsurge of Latin American immigration.) And Puerto Ricans, being US citizens, were always free to move to the mainland.
No doubt the fact that Mexico--unlike Germany or Italy--was next door, and that people in the Southwest had ready access to the Mexican press (and movies, radio, etc.) played a role. But the fact remains that without new immigration, Spanish would be in decline in the US. 75 percent of third-generation Hispanics who identify themselves as Hispanic say that English is their primary language--and 90 percent of third-generation Hispanics who do
not identify themselves as Hispanic (and there are quite a few of those) say so.
http://www.pewhispanic.org/2017/12/...fall-away/ph_2017-12-20_hispanic-identity_10/