In peacetime, it was relatively easy to rule the empire from its capital city, Rome. An eventual rebellion was expected and would happen from time to time: a general or a governor would gain the loyalty of his officers through a mixture of personal charisma, promises and simple bribes. A conquered tribe would rebel, or a conquered city would revolt. This would be a bad, but not a catastrophic event. The Roman legions were spread around the borders and the rebel leader would in normal circumstances have only one or two legions under his command. Loyal legions would be detached from other points of the empire and would eventually drown the rebellion in blood. This happened even more easily in cases of a small local native uprising as the rebels would normally have no great military experience. Unless the emperor was weak, incompetent, hated, and/or universally despised, these rebellions would be a local and isolated event.
During real wartime however, which could develop from a rebellion or an uprising, like the massive Jewish rebellion, this was totally and dangerously different. In a full-blown military campaign, the legions under the command of the generals like Vespasian were of a much greater number. Therefore a paranoid or wise emperor would hold some members of the general's family as hostages, to make certain of the latter's loyalty. In effect, Nero held Domitian and Quintus Petillius Cerialis the governor of Ostia, who were respectively the younger son and the brother-in-law of Vespasian. In normal circumstances this would be quite enough. In fact, the rule of Nero ended with the revolt of the Praetorian Guard who had been bribed in the name of Galba. It became all too obvious that the Praetorian Guard was a sword of Damocles, whose loyalty was all too often bought and who became increasingly greedy. Following their example the legions at the borders would also increasingly participate in the civil wars. This was a dangerous development as this would weaken the whole Roman Army.
The main enemy in the West were, arguably, the "barbarian tribes" behind the Rhine and the Danube. Augustus had tried to conquer them, but ultimately failed and these "barbarians" were greatly feared. But by and large they were left in peace, in order to fight amongst themselves, and were simply too divided to pose a serious threat.
The empire of Parthia, the arch-rival of Rome, at its greatest extent (c. 60 BC), superimposed over modern borders.
In the East lay the empire of Parthia (Persia). Crassus, a member of the First Triumvirate during the late republic, attempted an invasion in 53 BC, but was defeated by Persian forces led by Surena in the Battle of Carrhae. It was simply too far away to be conquered and then to be held. Any Parthian invasion was confronted and usually defeated, but the threat itself was ultimately impossible to destroy. Parthia would eventually become Rome's greatest rival and foremost enemy.
In the case of a Roman civil war these two enemies would seize the opportunity to invade Roman territory in order to raid and plunder. The two respective military frontiers became a matter of major political importance because of the high number of legions stationed there. All too often the local generals would rebel, starting a new civil war. To control the western border from Rome was easy, as it was relatively close. To control both frontiers, at the same time, during wartime, was somewhat more difficult. If the emperor was near the border in the East, chances were high that an ambitious general would rebel in the West and vice-versa. It was no longer enough to be a good administrator; emperors were increasingly near the troops in order to control them and no single Emperor could be at the two frontiers at the same time. This problem would plague the ruling emperors time and time again and many future emperors would follow this path to power.