The correct answer is: "the URSS".
The URSS and Nazi Germany signed the non-aggresion treaty known as Molotov-Ribbentrop Pact in 1939. The pact was broken in 1941, when Hitler attacked the URSS in the Operation Barbarrosa, initiating the fight on the Eastern European threatre motivated by the sucesses that, up to that point, Germany had accomplished in the Western front.
