the forgotten war and forgotten alliance - for Poland it meant annihilation while the good Polish allies France and UK stood by. War could have ended in 1939...
Soviet entry into the war in photos below
Untold massacres of Polish soldier and civilians, and then planned genocide ( Intelligenzaktion & AB Aktion) followed - the Germans killing Polish intelligentsia and upper levels of Polish society in German occupied Poland and merciless ethnic cleansing by deporting the millions of ethnic Poles from the Polish territories annexed to Germany into the GG,. The genocide carried in the Lasy Piasnickie in Pomorze was as huge as the Soviet massacres of the police and soldier prisoners in 1940-41.
While the Soviets were killing the Polish upper strata of society, they also killed policemen, veterans, farmers and merchants, they did lots of ethnic cleansing deporting more than a million Poles to Soviet Kazakhstan and Siberia. Soviet genocide of upper strata of Polish society is best exemplified by the Katyn Forest Massacre. (sadly the Jewish-American mayor of Jersey City wants to put away the finest monument to the victims of Katyn Forest Massacre, allegedly standing in path of Progress and too gruesome)
All this 1939-41 well before the killing of Polish Jews became a genocide ( after the German invasion of USSR). Germans created KL Auschwitz for Polish prisoners in 1940.
Not to mention looting of Polish art (more than 500,000 objects), Polish economy, including the finest Arabian horses in the world from Polish stud farms.