1919 - Reclaiming Grodo Governorate
Polands Reclamation Of The Kresy Region
Remembering that in the 3rd partition the Grodno Governorate was created and controlled by Russia, in World War 1 the "Grodno Governorate" became occupied by Germany (although it belonged to Russia) with the Polish citizens being persecuted by the Germans who were essentially "tenants" on land belonging to Russia.
At the end of World War 1 the Belarusian National Republic became independent from Russia. This lead to a Polish-Soviet war from 1919-1921. This war came to an end when the "Peace Treaty of Riga" came into effect at which point the region came to be known as the "Second Polish Republic" with Poland claiming rights to this region having won the Polish-Soviet war of 1919-1921.
Again, in simple terms, Polands reclaiming the Grodno Governate AKA Kresy region from Russia (as a result of Russia's war with Poland when Poland was fighting to protect land in which Poles, Lithuanians, Belarussians, Tartars and German colonists lived) was simply Poland attempting to re-establish old borders..
In 1921 Poland then rewarded Polish soldiers who had fought in World War 1 for Poland. They were given (sometimes sold) land in the "Kresy Region" which is currently western Belarus and western Ukraine. Roughly this equated to 20 hectares each. These people were called Osadniks meaning "Colonists" or "Settlers".
This must have hurt Russia's pride, to know that Poland had rewarded Polish soldiers with land that Russia had formerly regarded as its own.
This is a very simple view of the history of the region but one which helps us understand the backdrop of 1939.
The important point to take from this is that land that had originally belonged to Poland had been seized by Russia with Russia allowing the Germans to occupy it following which Poland reclaimed the land after World War 1.
You can imagine that both Germany and Russia were rather miffed at this and the contention must have started brewing at this point. It would take 20 years of this brewing before Russia would come back and invade the area in an attempt to re-seize Grodno Governorate (Kresy region) back for itself.