Complex sentence: Reagan never left the United States during the war, though he kept a film reel, obtained while in the service, depicting the liberation of the Auschwitz concentration camp, as he believed that someday doubts would arise as to whether the Holocaust had occurred.
Split into simpler sentences: Reagan never left the United States during the war, but he kept a film reel that he obtained while he was in the service. The reel depicted the liberation of the Auschwitz concentration camp; he believed that doubts would someday arise as to whether the Holocaust had occurred.