Short answers, pulled from the story.
The Soviet invasion of Poland began on the 17th of September 1939 and ended on the 6th of October 1939, lasting approximately twenty days. It concluded with the surrender of General Franciszek Kleeberg after the Battle of Kock.
The secret protocol of the Molotov-Ribbentrop Pact, signed on the 23rd of August 1939, divided Poland into German and Soviet spheres of influence. The territories east of the Pisa, Narev, Vistula, and San rivers were assigned to the Soviet Union.
The Polish Institute of National Remembrance announced in August 2009 that research estimates placed the number of people deported to Siberia and those who perished under Soviet wartime rule at around 150,000 Polish citizens in total. The Soviet NKVD carried out four major waves of deportations between 1939 and 1941.
The Katyn massacre was a Soviet mass execution in which the NKVD killed 22,000 Polish military personnel and civilians in 1940. When the burial pits were discovered in 1943, the Polish government demanded an independent examination, which led the Soviet Union to break off diplomatic relations with Poland.
A secret protocol of the British-Polish Mutual Assistance Pact of the 25th of August 1939 specified that the European power Britain was obligated to defend Poland against was Germany alone. France also refrained from action, calculating that overt denunciation of the Soviet Union would serve neither French nor Polish long-term interests.
Molotov declared on the 17th of September 1939 that the Polish state had ceased to exist and that the Soviet Union was acting to protect the lives and property of the populations of Western Ukraine and Western Belorussia. Soviet Politburo language styled the invasion a liberation campaign, a framing maintained in official publications throughout Soviet history until 1991.