A ranar 17 ga Satumba, Tarayyar Soviet ta shiga yankunan Poland da aka ba shi ta hanyar yarjejeniyar sirri na yarjejeniyar rashin cin zarafi daga gabas. A matsayin hujja don tabbatar da ayyukansu, Soviets sun ambaci rushewar Jamhuriyar Poland ta biyu kuma sun yi iƙirarin cewa suna ƙoƙarin taimaka wa mutanen Belarus da Ukraine. Ana ɗaukar mamayewar Soviet a matsayin sakamakon kai tsaye na yarjejeniyar, kodayake makarantar bita ta yi iƙirarin cewa ba haka lamarin yake ba kuma an yanke shawarar Soviet 'yan makonni bayan haka. Biritaniya da Faransa sun yi tir da matakin na Soviet, amma ba su sa baki ba. A musayar yankunan Poland da aka kama daidai da sharuɗɗan yarjejeniyar, tuni a ranar 17 ga Satumba Red Army da Wehrmacht sun gudanar da
wani fareti na soja na hadin gwiwa a Brest; Jamus ta koma hannun sojojin Soviet. A cikin yaƙe-yaƙe na gaba tare da sauran sojojin Jamhuriyar Poland ta Biyu, Tarayyar Soviet ta mamaye yankunan da suka dace da yankunan da ke da sha'awa, kamar yadda aka bayyana a cikin yarjejeniyar sirri ta sirri ga yarjejeniyar Molotov-Ribbentrop.