I'm very curious about this - was this an organized effort by Nazi Germany? Was it started by Weimar Germany, as the 20-year timeline given in the letter seems to suggest?

This claim has been made by a multitude of contemporary sources, few of which seem to go into any significant depth beyond the nebulous "Nazi Germany sent agents to the United States before the Second World War to attempt to learn various Native American languages." I am curious about their sources. This effort by the Germans could have occurred, either as individual anthropologists or linguists, organized teams, or government consultation of enthralled German students who had chosen to study in the United States and then returned to Germany. There also, on the other side of the Atlantic, was fear of Nazi infiltration into United States institutions before and during World War II (stoked by the existence of the German-American Bund, the most prominent American pro-Nazi social organization of the mid to late 1930s), and the work of a few, albeit innocent, researchers or students could have been blown out of proportion by startled military intelligence officials.

I wonder, also, whether this fact (like others), has started to be repetitively pushed down the line without any serious "re-research" (i.e., finding the original source, and then analyzing the content and interpretation of the primary documents used) into its origins, and has lost subtle details.

The conclusion of Frank Usbeck's 2015 book Fellow Tribesmen: The Image of Native Americans, National Identity, and Nazi Ideology in Germany, seems to suggest that this particular aspect of the Code Talkers' service (organized espionage campaigns by Weimar or, more particularly, Nazi Germany, against U.S. efforts to use Native American languages for military purposes), is not currently a well-researched area.

It is important, first, to understand the fact that the public in Weimar and Nazi Germany had a peculiar fascination with American "Wild West" and "frontier" culture and Native Americans, inspired by the works of German author Karl May.

The populations of both Weimar and Nazi Germany had a fascination with Native American and "Wild West" culture and imagery, born out of the cultural effects of the works of German novelist and short story writer Karl May (1842-1912), who produced a large volume of writings. He first began working in the mid-1870s, after being released from prison and recanting on a life of crime, and continued until his death. He frequently wrote using pseudonyms. His first publications were "travel stories," (accounts of his "visits" to other countries), but he later branched out into young adult and adult novels, often concerning the subjects for which he is most famous. He first began to gain notoriety in Germany in the 1890s.

May traveled to several places he had written about in his stories, including the United States, near the end of his life. He never actually visited the "Wild West," however, and died the year Arizona and New Mexico were admitted to the Union, regarded by some historians as the death knell of this era. He was able to paint such a heavily romanticized picture of this period that it "...has been so influential on several generations of German youth that Der Spiegel claimed recently that "May has advanced to be a kind of Praeceptor Germaniae ["father of Germany"], whose influence, without doubt, is greater than that of any other German author between Johann Wolfgang von Goethe and Thomas Mann." Many of May's tales are described as "cowboy and Indian" works, but the "cowboy" characters in these works can be more properly described as westmänner, a German term used by May to mean "frontiersmen" or "pioneers." The main consistent figure in May's novels, "Old Shatterhand," was:

...an American of German extraction, and a truly heroic Teutonic figure "Siegfried in a coonskin cap." Larger than life, Old Shatterhand was a powerful, blond, blue-eyed super man once described as a composite of "Hopalong Cassidy, Ulysses, Christ, Siegfried, and Kaiser Wilhelm" (Cracroft 1967: 257) who spoke 40 languages, including a fabricated Karl May dialect.

The vocabulary of this dialect used by Old Shatterhand and the westmänner was unlike any ever spoken in the West, a "type of Teutonized Texan," although it is difficult to conceive a Texan exclaiming (at any time in history!) "zounds," "woe to me," "'s death," or "pshaw and damnation, sir!" as Old Shatterhand was wont to do (Cracroft 1967: 254).

Old Shatterhand's companion was Winnetou, a "remarkable" Mescalero Apache chief. This relationship might have inspired the later stories of the Lone Ranger.

May had as an admirer the future leader of Nazi Germany, Adolf Hitler. As a boy, Hitler "seemed to be infected with the stories of Karl May and the Redskins." Hitler also (predictably) was fond of books on "German culture heroes and those on heroism in war."

To Adolf the adventures of Old Shatterhand and his comrades were almost an obsession. He tirelessly led his schoolmates into violent re-enactments and when the enthusiasm of the older boys flagged he recruited younger ones and even, on occasion, girls (Toland 1976: 14).

While the teacher was explaining new material, he read the books of Karl May...which he kept concealed under his desk. He would come to school with bowie knives, hatchets, and the like... (Langer 1972: 119).

Accounts of the relationships between May and Hitler as a child and in his later life appear only briefly in the works of many contemporary historians, and is only given a proper period treatment by Hitler's associate Albert Speer. The most telling aspects can be found in the writings of Hitler himself. He attached great significance to May's works, and said that they "were a primary force in the development of his thinking."

In 1943, the director of the Office of Strategic Services, William J. Donovan, enlisted a psychologist, Walter C. Langer, to compile a psychological analysis of Adolf Hitler. The influence of May's works on Hitler’s thinking was dismissed by Langer, as he considered them "simply 'Indian and Wild West stories'" and at the time, none of May's works had been translated into English and were only widely popular in Germany. In fact, Hitler continued to be a fan of May well into his rule of Germany. While being quoted as having said "I never read a novel. That kind of reading annoys me," he evidently placed May's books in a separate category. Hitler had a complete set of his works in his personal library bound in fine calfskin, a gift from Hermann Göring. According to Göring, Hitler kept these books close at hand:

They are much thumbed and read and usually one or two may be found in the small bedside bookcase with its green curtain in Hitler's bedroom (Oechsner 1942: 94-97).

In 1940, Klaus Mann, a German-born American writer and opponent of Adolf Hitler, postulated that May's works, through their influence on Hitler, were "influenc[ing] the history of the world:"

Being the militant incarnation of all good and noble principles, anything he did was necessarily good and noble: his cruelty was praised as heroism, his lack of morals interpreted as admirable ingenuity. The depraved, ambitious youth from Brunau was convinced that that was the way to bHe could see no reason why Old Shatterhand's convictions and tactics should not work if applied to national and international politics. One might conquer civilization by going back to the principles of the jungle...It hardly is an exaggeration to say that Karl May's childish and criminal fantasia has actually - though obliquely - influenced the history of the world.

Sources:

Cracroft, Richard H. "The American West of Karl May." American Quarterly 19, No. 2, Part 1 (Summer 1967): 249-258.

Mann, Klaus. "Karl May: Hitler's Literary Mentor." The Kenyon Review 2, No. 4 (Autumn 1940): 391-400.

Usbeck, Frank. Fellow Tribesmen: The Image of Native Americans, National Identity, and Nazi Ideology in Germany. New York City: Berghahn Books, 2015.

Wood, W. Raymond. "The Role of the Romantic West in Shaping the Third Reich." Plains Anthropologist 35, No. 132 (November 1990): 313-319.