The most infamous doctor at Auschwitz was Josef Mengele, the "Angel of Death", who worked in Auschwitz II from 30 May 1943, at first in the gypsy family camp. Particularly interested in performing research on identical twins, dwarfs, and those with hereditary disease, Mengele set up a kindergarten in barracks 29 and 31 for children he was experimenting on, and for all Romani children under six, where they were given better food rations. From May 1944, he would select twins and dwarfs during selection on the Judenrampe, reportedly calling for twins with "Zwillinge heraus!" ("twins step forward!"). He and other doctors (the latter prisoners) would measure the twins' body parts, photograph them, and subject them to dental, sight and hearing tests, x-rays, blood tests, surgery, and blood transfusions between them. Then he would have them killed and dissected. Kurt Heissmeyer, another German doctor and SS officer, took 20 Jewish children from Auschwitz to use in pseudoscientific medical experiments at the Neuengamme concentration camp. In April 1945, the children were killed by hanging to conceal the project.
German volunteers first used armour in live field conditions during the Spanish Civil War of 1936. Armour commitment consisted of Panzer Battalion 88, a force built around three companies of Panzer I tanks that functioned as a training cadre for Nationalists. The Luftwaffe deployed squadrons of fighters, dive bombers and transport aircraft as the Condor Legion. Guderian said that the tank deployment was "on too small a scale to allow accurate assessments to be made." The true test of his "armoured idea" would have to wait for the Second World War. However, the Luftwaffe also provided volunteers to Spain to test both tactics and aircraft in combat, including the first combat use of the Stuka.
Use of armoured forces was crucial for both sides on the Eastern Front. Operation Barbarossa, the German invasion of the Soviet Union in 1941, involved a number of breakthroughs and encirclements by motorised forces. Its goal according to Führer Directive 21 (18 December 1940) was "to destroy the Russian forces deployed in the West and to prevent their escape into the wide-open spaces of Russia." The Red Army was to be destroyed west of the Dvina and Dnieper rivers, which were about 500 kilometres (310 mi) east of the Soviet border, to be followed by a mopping-up operation. The surprise attack resulted in the near annihilation of the Voyenno-Vozdushnye Sily (VVS, Soviet Air Force) by simultaneous attacks on airfields, allowing the Luftwaffe to achieve total air supremacy over all the battlefields within the first week. On the ground, four German panzer groups outflanked and encircled disorganised Red Army units, while the marching infantry completed the encirclements and defeated the trapped forces. In late July, after 2nd Panzer Group (commanded by Guderian) captured the watersheds of the Dvina and Dnieper rivers near Smolensk, the panzers had to defend the encirclement, because the marching infantry divisions were still hundreds of kilometres to the west.
In June 1941 Germany attacked the Soviet Union and began the "Final Solution." Four mobile killing groups were formed called Einsatzgruppen A, B, C and D. Each group contained several commando units. The Einsatzgruppen gathered Jews town by town, marched them to huge pits dug earlier, stripped them, lined them up, and shot them with automatic weapons. The dead and dying would fall into the pits to be buried in mass graves. In the infamous Babi Yar massacre, near Kiev, 30,000-35,000 Jews were killed in two days. In addition to their operations in the Soviet Union, the Einsatzgruppen conducted mass murder in eastern Poland, Estonia, Lithuania and Latvia. It is estimated that by the end of 1942, the Einsatzgruppen had murdered more than 1.3 million Jews.