It took time to grow and was not constant; it had its roots in the Roman Republic, Hellenistic East, Carthaginian Africa and Celtic Europe (Part 2). The principal agency for the expansion from city-state to world-wide empire was the army (Part 3), which both followed and was followed by traders and merchants. The Roman army began as a citizen army under the Republic which was called out when need arose. As the needs grew and service lengthened, it developed into a permanent, professional army, trained and organized to a very high standard; providing it could choose its battleground, there were few other contemporary forces which could withstand its onslaught. Yet despite the extent of the Empire, the army never possessed more than thirty legions, although this number was considerably augmented with, at first non-citizen, auxiliaries of infantry and cavalry; even then its total manpower, at the height of the Principate, probably never exceeded much more than 350,000 men. At that same time, the frontiers, where almost all the army was ultimately stationed, stretched for nearly 10,000 km (6,000 miles), which, averaging out at about thirty-five men per kilometre, was no great concentration, especially since there was no central reserve; if one area was threatened, it could only be reinforced from another which was peaceful. Consequently, the chief external danger to the empire lay in one or more simultaneous attacks, which happened
with increasing frequency in the fourth and fifth centuries, and led to marked changes in strategy and organization.