Short answers, pulled from the story.
The term kresy first appeared in 19th century literature within poems by Adam Mickiewicz and Juliusz Słowacki. Zbigniew Gołąb traces its etymology to a medieval borrowing from the German word Grenze meaning borderline or circumscribed territory.
Soviet forces annexed Kresy territories on the 17th of September 1939 after Germany invaded Poland under the Molotov-Ribbentrop Pact. Significant portions of the ethnic Polish population faced deportation to Siberia and Kazakhstan within the Soviet Union during this period.
Over one million Poles relocated from the Eastern Borderlands to Recovered Territories formerly belonging to Germany between 1944 and 1946. Specific numbers included 150,000 from Wilno area, 226,300 from Polesia, 133,900 from Volhynia, and 618,200 from Eastern Galicia.
Topics including ecclesiastical architecture and Massacres of Poles in Wołyń faced publication bans due to Soviet propaganda reasons during Communist Poland. Official documents declared people born in Eastern Borderlands as born in the Soviet Union with very few themed books passing state censors.
Ethnic Poles constitute the largest minority group in Lithuania while Belarus hosts them as second-largest after Russians. Ukrainian census data shows 144,130 official Polish residents though some organizations claim numbers reach two million among assimilated populations.