Their story begins in late M40. There were already Orks on Xomia, leftover from a period long before human settlement. Xomia did not have any Imperial Guard at the time and so the PDF's solution to this problem was to keep them fighting amongst themselves and keeping them from threatening their settlements. When in the field they fought as guerrillas- killing Ork leaders, keeping them contained and fighting amongst themselves long enough for them to revert into savagery.