Rabbi Marvin Hier, founder and director of the Wiesenthal Center, says he is convinced that the four-page letter, acquired by the organization for $150,000 last month through a dealer, is genuine. “I am absolutely certain our copy is signed by Adolf Hitler,” Rabbi Hier said.

Rabbi Hier provided records indicating that the document was found in the final months of World War II by a U.S. Army soldier named William F. Ziegler. In a handwritten letter in 1988 provided by the dealer who sold the document to the Wiesenthal Center, Mr. Ziegler said he had found the document among others scattered on the floor of what appeared to be a Nazi Party archive near Nuremberg.

Rabbi Hier also provided documents from the dealer showing Hitler’s signature on the letter was validated in 1988 and again in 1990 by Charles Hamilton Jr., a New York handwriting expert and dealer who was famous for exposing fake Hitler diaries in 1983. Mr. Hamilton died in 1996.

Rabbi Hier said he had a chance to acquire the letter when it first came on the market in 1988, but was skeptical of the document because it was typed. That seemed odd to him for the period in question, when Hitler was an ordinary soldier in a country devastated economically by war. Typewriters were very costly in 1919 and even many military units did not have them. “How did he get hold of a typewriter?” Rabbi Hier asked.

This year, Rabbi Hier learned that there was a plausible explanation. In 1919, during the upheaval that followed Germany’s defeat in World War I, Hitler was attached to a military propaganda unit of the Bavarian Army in Munich that was trying to stamp out Bolshevik sentiment carried home by prisoners of war in Russia.

Hitler’s ability to hold the interest of his listeners drew him to the attention of a superior officer, Capt. Karl Mayr. When a soldier named Adolf Gemlich, who was doing similar propaganda work for the army in Ulm, wrote asking for a clarification of “the Jewish Question,” Captain Mayr gave Hitler the assignment.

Hitler wrote to Mr. Gemlich that occasional pogroms against the Jews were not enough — the Jewish “race” must be “removed” from Germany as a matter of national policy.