The German–Soviet Frontier Treaty was a second supplementary protocol,[1] of the Molotov–Ribbentrop Pact of 23 August.[2] It was a secret clause as amended on September 28, 1939, by Nazi Germany and the Soviet Union after their joint invasion and occupation of sovereign Poland.[3] It was signed by Joachim von Ribbentrop and Vyacheslav Molotov, the foreign ministers of Germany and the Soviet Union respectively, in the presence of Joseph Stalin.