William Patrick Hitler fled Nazi Germany when war broke out in 1939 to come and live with relatives in New York.
After being rejected from the US Army because of his family connection, he wrote an emotional plea to President Roosevelt in 1942 on why he so strongly wanted to fight for Allied Forces, the Daily Mail reported.
"I am the nephew and only descendant of the ill-famed Chancellor and Leader of Germany who today so despotically seeks to enslave the free and Christian people of the globe," he wrote in the letter.
"More than anything else I would like to see active combat as soon as possible and thereby be accepted by my friends and comrades as one of them in this great struggle for liberty," he added.
The letter made its way into the hands of then FBI Director J Edgar Hoover who approved Hitler for service against the Nazis.
William Hitler joined the Navy in 1944 but was removed from service three years later after being wounded.
William Patrick, who later changed his last name to Stuart-Houston, was the son of Adolf's half-brother Alois and first wife Bridget Dowling. He was born in Liverpool, England on March 12, 1911.
Three years later, Alois left his wife and son and took off for Europe. He remarried, becoming a bigamist, but kept in contact with his first wife.
He settled in Germany asking Bridget to send William for a visit. She agreed in 1929 when William was 18.
William had a half-brother Heinz Hitler, however he became a committed Nazi (and was later tortured to death by the Soviets).
In 1933, William went to Germany where he planned to cash in on his uncle's rise to fame.
In 1938, Adolf offered his nephew a high-power position within his organisation but William fled to Britain as he was suspicious it was a trap.
He escaped from Germany in 1939 with the help of a British spy and left for the US with his mother.
William died in 1987 at the age of 76 and was buried at the Holy Sepulchre Cemetery in Coram, New York.
The letter appears on the blog 'Letters Of Note' after it was included in the book, War Letters: Extraordinary Correspondence from American Wars.
