Complex sentence: Stangl had previously assisted in killing handicapped people during Operation T4 (the Euthanasia Programme of the National Socialists), and, before moving on to Treblinka, had been the first commandant of Sobibor.
Split into simpler sentences: Stangl had previously assisted in killing handicapped people during Operation T4 (the Euthanasia Programme of the National Socialists). Before moving on to Treblinka, he had been the first commandant of Sobibor.