In the sixth century A.D., while the Western Roman Empire had already succumbed to attacks by the Germanic tribes, the Eastern Empire stood its ground. In fact, thanks to a peace treaty with the Persians, Emperor Justinian the Great was able to concentrate on re-capturing territories belonging to the Western Empire. His generals, Belisarius and Narses, achieved a number of important victories against the Ostrogoths and the Vandals. However, their hope of re-establishing the Roman Empire was dashed by the arrival of what became known as the Plague of Justinian (541-542). According to Byzantine historian Procopius of Caesarea, this epidemic originated in China and Northeastern India and reached the Eastern Roman Empire via trade routes terminating in the Mediterranean. While modern scholarship was able to deduce that it was caused by the bacterium "Yersinia pestis", the same one that would later bring the Black Death, the single deadliest pandemic in human history, it remains uncertain how many actually died because of it. Current estimates put the figure between thirty and fifty million people, a significant portion of the human population at that time. The Plague arguably cemented the fate of Rome.