‘Wehrmacht' was the collective name for the armed forces of the Third Reich. Adolf Hitler was the commander in chief and all serving troops had to swear a personal oath of loyalty to him. He also reintroduced conscription in 1935. By organising the armed forces in this way, Hitler flouted the terms of the Treaty of Versailles, which had severely restricted the forces Germany could have. They were permitted, for example, to have fewer than 200,000 personnel, but by 1945 over 18,000,000 people had served over the ten year period the Wehrmacht had been in existence. Over five and a half million soldiers are estimated to have been killed during
WW2 and related campaigns. At the Nuremberg trials, the tribunal judged the Wehrmacht to have not been a criminal organisation per-se (unlike the SS), but this view has since been questioned. They were nonetheless found responsible for war crimes, many of which took place during the invasion of Poland in 1939, largely because they killed thousands of unarmed civilians. On the Eastern fronts, mass rapes by the military were common; an estimated 10,000,000 Soviet women were brutalised in this way, and many military brothels were established. At this stage, rape was not considered a war crime, so nobody at Nuremberg received punishment (a shameful waste of an opportunity – rape was not considered a war crime until UN resolution 1820 was ratified in 2008). The Wehrmacht was abolished by 1946, and both East and West Germany did not establish new armed forces until the mid 1950s.
Here is a clip from an interesting documentary series about the Wehrmacht: