The rare card surfaced at a family history roadshow almost a century after being sent by the future dictator to his comrade Karl Lanzhammer.
Recovering in the German city of Munich in December 1916 after suffering a leg wound in the Battle of the Somme, the then 27-year-old soldier wrote of his intention to "report voluntarily for the field immediately".
Historians say this demonstrates his attachment to his new network of army friends as much as his militaristic zeal.
Dr Thomas Weber, an expert on the period from the University of Aberdeen, said: "What's clear is Hitler desperately wants to return to the front and that's rather unusual, even for soldiers who were generally... read more