The Social Wars

Though the Cult of the Shattered Sky wasn't entirely crushed, as the cult dwindled in influence, many soldiers were allowed to return to their own cities. Often they found members of their family dead, having been accused during the crackdown. Many military units became so enraged at each other for such killings that they marched on each other without orders.
While the social wars didn't see many large scale battles, the imperial on imperial fighting resulted in massive losses of irreplaceable men, women and material and greatly undermined the central authority of Governor Alanus Damian. While the civilian populations largely escaped direct harm, the rebelling units often levied extra tax to fund themselves in their quest of retribution. None the less, a couple of towns and cities were captured and sacked.
The social wars generally aren't considered a civil war as none of the rebelling units declared their hostility against the central government. It was however the first time Imperial troops would fight each other on Ferra. The social wars lasted for just over 2 years, the rebelling units having depleted each other to such a point that the forces still following the orders of Governor Alanus Damian were able to disarm the last of the rebelling units without further bloodshed.