The correct answer is: "The Soviet Union"
The Soviet Union and Nazi Germany had signed the Molotov-Ribbentrop Pact in 1939. It was a nonaggresion and neutrality agreeement through which the two parties guaranteed not to undertake belligerent actions against each other or aid third parties to do so. When Germany invaded Poland in 1939, Stalin ordered the Soviet Union to do the same. At some point, both forces met and the the German–Soviet Frontier Treaty was signed, through which both countries would share this border during the early years of WWII.
The pact finished in 1942 when Nazi Germany launched Operation Barbarossa aiming to invade the URSS.
