Science Archaeology Ancient Roman mass grave shows its army's ethnic diversity Part of the empire's strength was drawn from its different populations. Breakthroughs, discoveries, and DIY tips sent every weekday. Among its many feats, Rome featured one of history's most organized and successful military forces . Its organization and deployment helped solidify the empire's centuries-long reign across the Mediterranean, Western Asia, North Africa, and Europe. In Croatia, archaeologists have discovered a mass grave dating back to the third century CE that indicates another key to Roman military strength: the diversity of its soldiers.