Answer:
The Reichswehr, (1919-1935) were the main military force of the Weimar Republic post WW1 and in the early days after the National Socialist takeover. They believed that by accepting the Treaty of Versailles the Weimar Republic were effectively stabbing the German people in the back.
Explanation: (Extra info, not necessary to read) A cool bit of info, albeit categorically false, after the end of WW1 a very popular myth spread through Germany known as the Dolchstoßlegende (Not sure if the German is right but it means the “back stabber myth”). It was an antisemitic idea held by some that the loss of WW1 was actually not caused by military losses by the Imperial German Army, but instead actually of Jewish and communist/Bolshevik conspirators who wanted to bring down Germany from within. It branded the entire government, specifically the officials who took part in signing the treaty of Versailles, the “November Criminals”.