September 28, 1939 – Germany and the Soviet Union partition Poland

On September 28, 1939, as their joint invasion of Poland was winding down, Germany and the Soviet
Union, acting on Stalin’s proposal, agreed to make changes to
their respective spheres of influence as set forth in the Molotov-Ribbentrop
pact.  In the revised treaty, Germany relinquished to the Soviet Union its
claim to a sphere of influence on Lithuania
in exchange for the Soviet Union relinquishing to Germany
its sphere of influence to sections of central Poland,
including Warsaw and Lublin. 
On October 8, 1939, Germany
annexed western Poland,
including Danzig, the Polish Corridor, and Silesia,
and established the German-run General Governorate in the rest of the German-assigned
territory in Poland.

The Soviet Union also annexed its share of Polish
territories, partitioning them among its subordinate states Belarus, Ukraine
and Lithuania,
and implementing Sovietization policies in ethnic Polish-majority regions.

In German-controlled Poland, which was extended to include
all of Poland after German forces captured the Soviet section of Poland in the
early stages of Operation Barbarossa (the German invasion of the Soviet Union)
in June 1941, Nazi Germany implemented policies aimed at achieving Lebensraum,
where ethnic Germans would settle in the former Polish territories which then
would be completely Germanized politically, economically, socially, and
culturally.  As Lebensraum entailed displacing
the native populations, Generalplan Ost (General Plan East) was initiated in a
series of programs of depopulating, resettling, or otherwise eliminating the
Polish population from lands that were destined to become fully German.  Central to Nazi doctrine was the concept of
German racial superiority, and that German ethnic purity was to be maintained
and not tainted by the blood of races which the Nazis classified as inferior
(Untermensch, or sub-human), which included Poles and other Slavic peoples,
Jews, and Roma (gypsies), among others.

The colonization and full Germanization of Polish
territories were to be accomplished in stages over many years.  But of more urgency to the Germans was the
fate of Polish Jews, whose eradication was determined in January 1942 through
the euphemistically called “Final Solution”. 
In the aftermath of the Polish campaign, German authorities segregated
the three million Polish Jews, who were then forced into the hundreds of Jewish
ghettos quickly set up across Poland.  In the ensuing period, Polish and other Jews
across Europe were transported by train to
specially constructed labor, concentration, and extermination camps where the
mass executions ultimately were carried out. 
Aside from Jews, Slavs, and Roma, Nazi extermination policies also
targeted the physically and mentally disabled, homosexuals, political
opponents, communists, prisoners of war, resistance fighters, and other groups.

In Poland, as a result of the German occupation, some six million Poles perished, or 20% of the total population.  Of this number, three million were Jews, of whom 90% were killed. (Taken from Wars of the 20th Century – World War II in Europe)