Molotov%E2%80%93Ribbentrop Pact
Most notably, there was also a secret protocol to the pact, revealed only after Germany's defeat in 1945, although hints about its provisions were leaked much earlier, e.g., to influence Lithuania. According to said protocol Romania, Poland, Lithuania, Latvia, Estonia and Finland were divided into German and Soviet "spheres of influence". In the north, Finland, Estonia and Latvia were assigned to the Soviet sphere. Poland was to be partitioned in the event of its "political rearrangement"—the areas east of the Pisa, Narev, Vistula and San rivers going to the Soviet Union while Germany would occupy the west. Lithuania, adjacent to East Prussia, would be in the German sphere of influence, although a second secret protocol agreed to in September 1939 reassigned the majority of Lithuania to the USSR. According to the secret protocol, Lithuania would be granted the city of Vilnius – its historical capital, which was under Polish control during the inter-war period. Another clause of the treaty was that Germany would not interfere with the Soviet Union's actions towards Bessarabia, then part of Romania; as the result, Bessarabia was joined to the Moldovan ASSR, and become the Moldovan SSR under control of Moscow.

Q: What rivers would the soviet union claim as their own during the invasion of poland?
Pisa, Narev, Vistula and San rivers