Short answers, pulled from the story.
The Red Army crossed the border into Poland on the 17th of September 1939. Marshal Semyon Timoshenko commanded the invasion on the Ukrainian Front while General Mikhail Kovalyov led forces on the Belarusian Front.
The pact contained secret protocols regulating detailed plans for the division of states in northern and eastern Europe into German and Soviet spheres of influence. The agreement allowed Germany and the Soviet Union to partition Poland with territories east of the Pisa Narev Vistula and San rivers falling to the Soviet Union.
Research announced in August 2009 estimates that around 150,000 Polish citizens perished under Soviet wartime rule or were sent to Siberia and other remote parts of the Soviet Union. The NKVD killed 22,000 Polish military personnel and civilians in the Katyn massacre in 1940 alone.
Britain and France refrained from critical reaction to the Soviet invasion and annexation of Eastern Poland since neither expected confrontation with the Soviet Union at that time. French forces advanced tentatively into the Saar region in early September but retreated behind the Maginot Line upon Polish defeat on the 4th of October.
The Soviet Union signed the Polish-Soviet border agreement of August 1945 with new internationally recognized Polish Provisional Government of National Unity on the 16th of August 1945. This agreement recognized status quo as new official border between two countries except for region around Białystok and minor part of Galicia east of San River around Przemyśl later returned to Poland.