By mid-September, the German army had invaded most of the Polish territories. Although Polish armed forces, encircled by the superior forces of Wehrmacht, continued combat operations all over Poland. A number of Polish counter-offences rendered a temporary success. The defenders of Warsaw fought until September 22, 1939.
The Red Army entered Poland under the pretext of the protection of the non-Polish national minorities in Eastern Poland. In reality it was a planned operation, provisioned by the Secret Protocol signed on August 23, 1939 and attached to the Soviet-German Non-Aggression Pact. A precondition to the Soviet invasion of Poland had to be the fall of Warsaw. German government communicated to Stalin that Warsaw had fallen, although it was not true.
In geopolitical sense, the German and Soviet invasion of Poland meant the Forth Partition of the country. Both Hitler and Stalin had settled their old scores with the independent Poland. The Soviet zone of occupation included Eastern Galicia, Western Volhynia and Polesije regions.
These ethnic Ukrainian territories also contained the large Jewish population, approximately 800,000 people. A small number of Jews returned back to the German-occupied Poland, namely to the General Government. However, the majority remained in the Soviet-controlled territories, which by the end of 1940 were officially incorporated in the USSR.
It has to be said that manifold more Jews survived in the Soviet interior than in German-occupied Poland. Many of them later joined the Anders Army or the First Polish Division named after Kosciuszko. This division would become a core of the pro-Soviet forces that eventually took control over the liberated Poland in 1944 – 1945.
The Organization of Ukrainian Nationalist continued to resist against the all might of the Soviet military since 1944 and until 1956. Western Ukraine, the former Eastern Galicia, that once was a home to Poles, Ukrainians and Jews, preserved its role as a Ukrainian Piedmont until Ukraine gained independence in 1991. It is still regarded to be a Ukrainian Piedmont. Only after the Soviet Perstroika, the Secret Protocol between Nazi Germany and the USSR was made public.