 On 1 September 1939 , Germany invaded Poland , initiating World War II in Europe , and on 17 September , pursuant to the <unk> @-@ <unk> Pact , Poland was invaded by the Soviet Union . Subsequently Poland was partitioned again – between these two powers – and remained under occupation for most of the war . By 1 October , Germany and the Soviet Union had completely overrun Poland , although the Polish government never formally surrendered , and the Polish Underground State , subordinate to the Polish government @-@ in @-@ exile , was soon formed . On 8 October , Nazi Germany annexed the western areas of pre @-@ war Poland and , in the remainder of the occupied area , established the General Government . The Soviet Union had to temporarily give up the territorial gains it made in 1939 due to the German invasion of the Soviet Union , but permanently re @-@ annexed much of this territory after winning it back in mid @-@ 1944 . Over the course of the war , Poland lost over 20 % of its pre @-@ war population amid an occupation that marked the end of the Second Polish Republic . 
