Complex sentence: In 1944, the advancing Soviets captured the camp and reopened it to include mostly members of the Home Army--a Polish resistance movement, which were treated by Soviets as hostile to due to its loyalty to the Polish government in exile.
Split into simpler sentences: In 1944, the advancing Soviets captured the camp and soon afterwards reopened it. The prisoner population of the newly reopened camp were mostly members of the Home Army--a Polish resistance movement, treated by Moscow as hostile to the Soviet authorities due to its loyalty to the Polish government in exile.