Complex sentence: With the beginning of the World War II in September 1939 the castle was ransacked and in August 1944 the local German authorities looted the oldest surviving monuments of Polish culture, some of them were retrieved by the Polish Government after the war.
Split into simpler sentences: With the beginning of World War II in September 1939, the castle was ransacked. In August 1944, the local German authorities looted the oldest surviving monuments of Polish culture, some of which were retrieved by the Polish Government after the war.