But also Poland signed the Protection of Minorities Treaty which stipulated that the State of Poland was establish on the part of the Russian Empire inhabited by Poles.
Uhmmm i never heard about that. Could you please provide more info?
Poland signed the 1919 League of Nations' supplemental Treaty of Versailles on the treatment of minorities or as I called it Protection Minorities Treaty. That Treaty stated that Polish State was established on the territory of the former Russian Empire inhabited mainly by Poles. Poland undertook to protect minorities living within her borders i.e. (in 1939) 15% Ukrainians, 8% Jews, 4% Belorussians, 3% Germans. In the Eastern regions of Poland (captured after Russian-Polish war) those minorities constituted the majority of the Polish citizens.
In 1934 the Polish foreign minister Joseph Beck announced the unilateral abrogation of that Treaty in Geneva. That abrogation caused the protests of UK and France. I would never claim that Russia and Germany never used the complicated situation with ethnic groups in Poland but everybody was pursuing for its own interests. In 1937 Germany forced Poland to sign the Agreement on German minority in Poland (Gdansk/Dantzig).
But Poland in her turn delivered an ultimatum to Lithuania in 1938 demanding to protect the rights of Polish minority in Vilno and protesting the decision to call Vilno the capital of Lithuania or Polish army would occupy Lithuania. Russia bulldozed Poland and forced it to retreat.
So in 1939 when Russia returned the captured lands she formally did not do anything wrong.
..except for breaking non-aggression treaty with Poland and of course treaty in Riga, where Soviets recognized Polish eastern border.
About the Riga treaty. Do not forget that Poland abrogated her own commitments taken in Spa to define the Eastern border along the Kerzon line. So the score was even. As for the non-aggression treaty the Polish state did not exist when Russian army moved into its former territory. No Polish government was present.
Do not forget that in 1938 Poland unilaterally abrogated the Polish-Czech non-aggression treaty and presented an ultimatum to the Czechoslovakian government to secure the rights of Polish minority in Teshen region and then immediately occupied the Tenesh Silesia inhabited by Poles.
Besides Stalin exploited the right of nations to self-determination proclaimed by the League of Nations (just like Hitler in Czechoslovakia). Ukrainians in Galicia and belorussians in the Western Byelorussia expressed their wish to join their "Soviet brothers".
And Poles in Wilenszczyzna too, of course, as well as Poles in Grodno (i don't have to remind of course what was happening in Grodno (Hrodno today) in 1939)
If we speak about Grodno then in 1939 the population of Grodno constituted about 47-49 thousand people, 30 thousand of them were Jews (more then 60%), 5-6 thousand of them were Poles and others were Belorussians. So in 1939 the remnants of Polish Army fiercely opposed the Russian attacks in Grodno. Do you really believe that they were supported by the majority of the local population? Come on, give me a break. Shall we start with the Jewish situation in interwar Poland? Yes sure, Russian did not like them better but they did not know that yet.