Complex sentence: The river is within the area of the Ordensstaat which was established in the 13th century by the Teutonic Knights, later known as East Prussia, a part of Germany until 1945 when it was partitioned among Poland and the Soviet Union.
Split into simpler sentences: The river is within the area of the medieval Ordensstaat, which was established in the 13th century by the Teutonic Knights. This territory became later known as East Prussia, a part of Germany until 1945 when it was partitioned between Poland and the Soviet Union.