At its territorial height in the first and second century CE, the Roman Empire may have contained between 45 million and 120 million people. The Roman military can be viewed as one of the greatest armies in world history. Historian Edward Gibbon estimated that "the size of the Roman army most probably formed a standing force of three hundred and seventy-five thousand men, at the Empire's territorial peak in the time of the Emperor Hadrian." The Roman population slowly decreased because many barbaric tribes settled along the borders of the Empire and began sacking cities and attacking villages. By the end of the fourth and the beginning of the fifth century, the
Roman military began to decay into tons of problems. Many factors contributed to the military decline of the Roman Empire, including a lack of discipline, financial problems, and poor military tactics and decisions by the empire. In result of these factors, barbarians were able to attack the walls of the empire with more ease because of the

lack of defense. These barbarian attacks helped prone the efficiency of the army's defenses and tactics which impart the major downfall of the empire. A change in military tactics during the third and fourth century CE was central in the decline of the Roman
Army. This left commanders and soldiers confused, this resulted as well in casualties and loses in war. Because of a controversial change in tactics and weapons, the
Romans were virtually useless against these barbaric tribes. Thus, a change in these tactics may lie seen as a major scene of the decline of the Roman Empire.

During the Pax Romana, bustling trade flowed over routes patrolled by Roman legions and ships. Rome’s treasuries were enriched by gold and silver taken from conquered territories. Most important of all, the empire’s farms grew enough grain to feed the population of the cities.