What if the Plague of Justinian, that wiped out a third of the Eastern Empire's population, did not occur? Would the empire have thus had the manpower and economic stability to launch a full reconquest of the Roman world? Even with the plague Justinian's generals conquered all of Italy, North Africa, and parts of Iberia. Perhaps without it they could have fought the Franks and Visigoths and reconstituted the empire to the Rhine.