The German–Soviet Frontier Treaty was a second supplementary protocol, of the Molotov–Ribbentrop Pact of 23 August. 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. 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. Only a small portion of the protocol, which superseded the first treaty, was publicly announced while the spheres of influence of Nazi Germany and the Soviet Union remained classified. The third secret protocol of the Pact was signed on January 10, 1941 by Friedrich Werner von Schulenberg, and Molotov, whereas Germany renounced its claims to portions of Lithuania, only a few months before their anti-Soviet Operation Barbarossa.