The ideology of Nazism brought together elements of antisemitism, racial hygiene, and eugenics, and combined them with pan-Germanism and territorial expansionism with the goal of obtaining more Lebensraum (living space) for the Germanic people. Nazi Germany attempted to obtain this new territory by attacking Poland and the Soviet Union, intending to deport or kill the Jews and Slavs living there, who were considered inferior to the Aryan master race.  Mengele joined the Nazi Party in 1937 and the Schutzstaffel (SS; protection squadron) in 1938. He received basic training in 1938 with the Gebirgsjager (mountain infantry) and was called up for service in the Wehrmacht (German armed forces) in June 1940, some months after the outbreak of World War II. He soon volunteered for medical service in the Waffen-SS, the combat arm of the SS, where he served with the rank of SS-Untersturmfuhrer (second lieutenant) in a medical reserve battalion until November 1940. He was next assigned to the SS-Rasse- und Siedlungshauptamt (SS Race and Resettlement Main Office) in Posen, evaluating candidates for Germanisation.  In June 1941, Mengele was posted to Ukraine, where he was awarded the Iron Cross Second Class. In January 1942, he joined the 5th SS Panzer Division Wiking as a battalion medical officer. He rescued two German soldiers from a burning tank and was awarded the Iron Cross First Class, as well as the Wound Badge in Black and the Medal for the Care of the German People. He was seriously wounded in action near Rostov-on-Don in mid-1942, and was declared unfit for further active service. After recovery, he was transferred to the Race and Resettlement Office in Berlin. He also resumed his association with von Verschuer, who was at the Kaiser Wilhelm Institute of Anthropology, Human Heredity, and Eugenics. Mengele was promoted to the rank of SS-Hauptsturmfuhrer (captain) in April 1943.

Answer this question "When did he join the military?" by extracting the answer from the text above.
Mengele joined the Nazi Party in 1937 and the Schutzstaffel (SS; protection squadron) in 1938.