The Nivelian Civil War occurred some time between 3589 and 3624 A.D. The cause of the war is unknown. However, as a result, the then newly settled Mido system declared independence from the Nivelian Republic and formed the Mido Confederation of Planets along with some Terran rebels and outlaws. Although it is also unknown how much the Terran Federation was involved in the war but as of today, they have accepted the declaration of independence from the Mido Confederation but the Nivelian Republic still claim the Mido system as its own and there are still many skirmishes along the Mido-Nivelian border.
This conflict comes info play during the Supernova DLC as there will be a supernova in the Mido system area, and the Nivelians are needed to help evacuate the Midorians. But old habits die hard, and may lead to the prevention of saving the civilians.