Washington Heights, in Manhattan, had many German immigrants in the 1930s and 1940s. The neighborhood was nicknamed “Frankfurt on the Hudson” (cited in print from only the 1980s). Washington Heights has been mostly Dominican since the 1980s-1990s and the “Frankfurt on the Hudson” nickname is no longer used.
 
Another nickname for the same German community—“The Fourth Reich”—has been cited in print since the 1930s.
 
       
Wikipedia: Washington Heights, Manhattan
Washington Heights is a New York City neighborhood in the northern reaches of the borough of Manhattan. It is named for Fort Washington, a fortification constructed at the highest point on Manhattan island by Continental Army troops during the American Revolutionary War, to defend the area from the British forces. Washington Heights borders Harlem to the South, along 155th street, Inwood to the North along Dyckman Street, the Hudson River to the West and Harlem River to the East.
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History
The Battle of Fort Washington, which occurred on November 16, 1776, saw Fort Washington fall to the British at great cost to the American forces; 130 soldiers were killed or wounded, and an additional 2,700 captured and held as prisoners, many of whom died on prison ships anchored in New York Harbor. The British renamed it “Fort Knyphausen” to honor the German general who had led the successful attack, and held it for the remainder of the war. The progress of the battle is marked by a series of bronze plaques along Broadway.
 
The series of ridges overlooking the Hudson were sites of villas in the 19th century, including the extensive property of John James Audubon.
 
In the early 1900s, Irish immigrants moved to Washington Heights. European Jews went to Washington Heights to escape Nazism during the 1930s and the 1940s. During the 1950s and 1960s, many Greeks moved to Washington Heights; the community was referred to as the “Astoria of Manhattan.” By the 1980/90s, the neighborhood became mostly Dominican.
 
By the 2000s, after years when gangsters ruled a thriving illegal drug trade, urban renewal began. Many Dominicans moved to Morris Heights, University Heights, and other west Bronx neighborhoods. While gentrification is often blamed for rapid changes in the neighborhood, the changes in population also reflect the departure of the dominant nationality. Even though Dominicans still make up 73 percent of the neighborhood, their moves to the Bronx have made room for Mexicans and Ecuadorians, according to The Latino Data Project of the City University of New York. The proportion of whites in Washington Heights has declined from 18 percent in 1990 to 14 percent in 2005.
 
Wikipedia: Hudson Heights (Manhattan)
Hudson Heights is a neighborhood in Upper Manhattan, New York City in the United States. Hudson Heights is a sub-neighborhood of Washington Heights. Its name is the combination of its proximity to the Hudson River, and its geographical altitude, which includes the highest natural point in Manhattan. Hudson Heights is bounded by the Hudson River to the West, Broadway to the East, 173rd street to the South, and Fort Tryon Park to the North. The name Hudson Heights, and the boundaries associated with the neighborhood, were first established by the Hudson Heights Owners Coalition, which was founded in 1993.
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Frankfurt-on-the-Hudson
In the years after World War II, the neighborhood was referred to as Frankfurt-on-the-Hudson for the dense population of German and Austrian Jews who had settled there. A disproportionately large number of Germans who settled in the area had come from Frankfurt-am-Main, possibly giving rise to new name. No other neighborhood in the city was home to so many German Jews, who had created their own central German world in the 1930s.
 
So cosmopolitan was that world that in 1934 members of the German-Jewish Club of New York started Aufbau, a newsletter for its members that grew into a newspaper. Its offices were nearby on Broadway. The newspaper became known as a “prominent intellectual voice and a main forum for German Jewry in the United States,” according to the German Embassy in Washington, D.C. “It featured the work of great prominent writers and intellectuals such as Thomas Mann, Albert Einstein, Stefan Zweig, and Hannah Arendt. It was one of the only newspapers to report on the atrocities of the Holocaust during World War II.”
 
In 1941 it published the Aufbau Almanac, a guide to living in the United States that explained the American political system, education, insurance law, the post office and sports. After the war, Aufbau helped families that had been scattered by European battles to reconnect by listing survivors’ names. Aufbau‘s offices eventually moved to the Upper West Side. The paper nearly went bankrupt in 2006, but was purchased by Jewish Media AG, and exists today as a monthly news magazine. Its editorial offices are now in Berlin, but it keeps a correspondent in New York.
 
The 1985 film We Were So Beloved tells the stories of neighborhood Jews who escaped the Holocaust. When their children grew up they tended to leave the neighborhood and, sometimes, the city. By 1960 German Jews accounted for only 16% of the population in Frankfurt-on-the-Hudson.
 
The neighborhood became less Jewish into the 1970s as Soviet immigrants moved to the area. Later, families from the Caribbean, especially Puerto Rico and the Dominican Republic, made it their home. (So many Dominicans live in Washington Heights that candidates for the presidency of the Dominican Republic campaign in parades in the 150s and 160s.) In the 1980s African-Americans started moving in, followed shortly by other groups. “Frankfurt-on-the-Hudson” no longer described the area.
 
New York (NY) Times
Movie Review
THE SCREEN: ‘WE WERE SO BELOVED’
By VINCENT CANBY
Published: August 27, 1986
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At first Mr. Kirchheimer’s focus seems short. His subjects are members of his family and friends who were among those able to leave Germany before 1939 and who settled in Washington Heights. There, on the upper tip of Manhattan, they created a solid, prosperous middle-class community, sometimes known as Frankfurt-on-the-Hudson, while millions of others were being herded into the death camps.
   
Google Books
The Dr. Ruth phenomenon:
The sexual awakening of America?
By Barbara Multer-Wellin
New York, NY: Richardson & Steirman : Distributed by Kampmann
1987
Pg. 21:
She moved into an apartment in a section of Manhattan called Washington Heights, and nicknamed the Fourth Reich and Frankfurt-on-the-Hudson.
 
OCLC WorldCat record
Frankfurt on the Hudson : the German Jewish community at Washington Heights, 1933-83, its structure and culture
Author: Steven M Lowenstein
Publisher: Detroit, MI : Wayne State University Press, 1989.
Edition/Format:  Book : English
 
New York (NY) Times
The Last of Frankfurt-on-the-Hudson; A Staunch, Aging Few Stay On as Their World Evaporates
By JAMES BENNET
Published: August 27, 1992
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People that lost almost everything in Europe remade a section of Manhattan into a familiar hometown. Through the growing immigrant network, many of the women, like Mrs. Goldschmidt, found work as nannies or housekeepers, while the men became bookkeepers, tailors and shop owners. Mrs. Golschmidt’s husband worked as a hotel busboy. By outsiders, the area was dubbed the Fourth Reich or Frankfurt-on-the-Hudson.
 
No one calls it that anymore.