Short answers, pulled from the story.
The Neumark, also called the New March or East Brandenburg, was a region east of the Oder River that belonged to the Margraviate of Brandenburg and its successor states. It became part of Poland in 1945, except for some villages west of the Oder that remained in Germany.
The Neumark became part of Poland after the Potsdam Conference following World War II, when the majority of the region was assigned to Polish administration starting in 1945.
Margrave John, a younger son of Joachim I Nestor, ruled the Neumark as the independent state of Brandenburg-Küstrin from 1535 to 1571. It then returned to his nephew, Elector John George.
Brandenburg pawned the Neumark to the Teutonic Knights in 1402 after losing interest in the territory once the Ascanian dynasty died out in 1320. The Knights gained full control by 1429, and Frederick II of the Hohenzollern dynasty eventually reacquired the territory in 1463 for 40,000 guilders.
The name Neumark, meaning New March, came into use after the middle of the 15th century, describing the territory as a new eastern extension of the Margraviate of Brandenburg across the Oder.
The 1939 census counted 645,000 residents in the Neumark, including 3,000 non-Germans. After the war, Germans were expelled under the Potsdam Agreement, and only about 5,000 of the pre-war inhabitants remained in the region by 1950.
Most of the former Neumark lies within Poland's Lubusz Voivodeship, with the northern towns of Choszczno, Myślibórz, and Chojna in the West Pomeranian Voivodeship. A small area near Cottbus became part of Germany after reunification in 1990.