Complex sentence: A rising by the Aliab Dinka in 1919 was put down harshly in 1920, and that year the Nuer were attacking both the Dinka and the "Burun" peoples on the border with Ethiopia.
Split into simpler sentences: A rising by the Aliab Dinka in 1919 was put down harshly in 1920. That year the Nuer were attacking both the Dinka and the Burun people on the border with Ethiopia.