Rome is thought of as the prototype of an imperial power.
Starting as a small city state on the Tiber River in central Italy, it became in four stages one of history’s great empires.
(1)  Gained control of the Italian peninsula
(2)  Became the major power in the western Mediterranean basin by defeating Carthage
(3)  Conquered much of the eastern Mediterranean (the Hellenistic world) from the heirs of Alexander the Great
(4)  Absorbed a good part of northern Europe
Most expansion occurred while Rome was still a Republic.  Conquests under the Empire involved only periferal areas, like Britain and Dacia.
Civil Wars:  Beginning around 130 BCE and dragging on for generations, these bloody wars put an end to the Roman Republic and issued in the Roman Empire. Theoretically, the wars pitted a popular party against an aristocratic or senatorial party, but increasingly they degenerated into a mere power struggle between generals on each side.
Political generals:  men who were willing to use their military power to seize control of the state.
Julius Caesar: The prototype political general who defeated all of his major rivals.  Only his assassination in 44 B. C. prevented him from ending the civil wars that continued for another 17 years.
Octavian (later renamed Augustus Caesar):  Caesar's grandnephew  who restored to the Roman world in 27 BCE.  With the establishment of his authoritarian rule, Rome ceased to be a republic and became what we call the Roman Empire; with Octavian as the first emperor.
What changes made Rome an empire?  The appearance of an emperor.
Battle of Actium (31 BCE):  Octavian defeated his last major rival, Mark Anthony, along with Anthony's ally and lover, Queen Cleopatra of Egypt, both of whom committed suicide.
After four years of "mopping up" what opposition remained, he returned to Rome and resigned his emergency powers.  After a hero’s welcome, Octavian received from the pliant Roman Senate the permanent powers that would define the emperor.
Augustus Caesar:  New name conferred upon Octavian by the Senate.  Augustus, meaning the revered one, combined with his family name, Caesar.
Augustan Compromise:  Term referring to the new balance between authoritarianism and republicanism instituted by the first emperor.  While preserving the semblance of many republican traditions, Augustus increasingly concentrated power in his own hands. The emperor retained many of the more cherished institutions of the Republic, including the Roman Senate which embodied the traditions of the past and many of the executive offices (consul, tribune, censor, etc.) that had existed under the Republic.  In theory, his new government became a partnership between the emperor and the Senate in ruling the Roman world.  In reality, the emperor was always the dominant partner.
Maior:  Literally “greater.” the authority of all important offices in the Roman state also belonged to the emperor; his authority in each was declared to be “greater” than that of the office-holder.
The government of the Republic had been largely retained, but now a permanent “super-authority” known as the emperor had been added whose power was greater than the rest of the structure.
Dictator:  early Roman official, appointed by the Senate in an emergency, who would hold supreme power for six months, then resign that power back to the Senate.  To an extent, this temporary official served as precedent for the imperial office.
Princeps:  the word, which means "first citizen," was the preferred title of Augustus and his successors.  (From it, we derive the modern word "prince.")
Imperator:  a Roman military title, conferred upon Augustus and his successors  which translates as roughly "commander of the army" or “commander in chief.  Fom it, the word "emperor" is derived.
Pax Romana (literally the 'Roman Peace'):  a prolonged period of peace, stability, and prosperity which most historians regard as the highpoint of Roman civilization.  It began in 27 B. C. when Augustus seized power and ended around 180 A. D., with the death of the emperor, Marcus Aurelius.
At the height of the Pax Romana, the Roman Empire was divided into 45 provinces, containing a population of approximately 75 million inhabitants.  This was probably the largest population of any state in the ancient world, one which would rarely if ever be equalled until recent centuries.  (The Mongol Empire of the Middle Ages may hav been an exception.)
Roman military capacity:  A major factor making possible the Roman peace.  During this period, the vast Roman army, consisting of some thirty legions, each of which was accompanied by large numbers of auxiliary troops, put down the few internal disturbances and guarded the frontiers against potential invaders. First line troops were apportioned among the frontier provinces and a few particularly troublesome internal regions such as Palestine.  Adding the imperial guard (the Praetorians) and the sailors in the fleet brought the total of regular troops to over a half million.  Territorial militias would bring that to over a million. Protecting the empire throughout the Pax Romana was this seemingly invincible military system with Roman legions stationed at strategic points, and massive stone walls built to keep out the barbarians.  Behind Rome’s well-guarded borders, most of the inhabitants lived a tranquil existence most of the time.
Augustus inherited the military establishment based on the legion that had been built up under the Republic and honed in the civil wars.  He now fine-tuned it to be more reliable, not only to better protect the empire, but to discourage competitors:
Augustan military reforms:
(1)    Placed the Roman army under what amounted to personal control by appointing all major officers
(2)  Stationed regular army in imperial rather than senatorial provinces
Imperial provinces:  those under direct imperial control, ruled by governors hand picked by Augustus, such as Spain, Britain, Syria (including Palestine), and the Rhineland.  Since these were usually trouble spots, the Since the army was concentrated in these imperial provinces, giving Augustus a virtual monopoly on military authority within the state.
Senatorial provinces:  provinces ruled in the old way through governors appointed by the senate.  These included the most peaceful provinces of the Empire such as Sicily, Greece, north Africa, and Asia Minor.  Legions sent into senatorial provinces were not commanded by the governor who had only a bodyguard and territorial militia.
(3)  Converted the army from an instrument of conquest into an instrument of defense. Believing that further conquest was unnecessary and might even be harmful to Rome, Augustus redesigned the Roman military to defend existing borders rather than expand them. For the most part, the emperors of the Pax Romana adopted the defensive strategy..
Few significant departures from a defensive strategy during the Pax Romana and these were the exception:
(a)  The first emperor’s attempt to invade Germany met with disaster at the battle of Teutoberg Forest when overconfident force marched into a trap set by the first hero in German history, Arminius Hermann (9 CE).  
(b)  Conquest of Britain by the emperor Claudius (c. 50)
(c)  Conquests of Trajan (the most expansion-minded emperor), in the east (early 2nd century); only Dacia was retained by his successor  .
Dacia:  Territory north of the Danube; although it remained Roman for only two centuries, it became so Romanized that today we still call it Rumania, or "land of the Romans".  The extent of Roman influence is illustrated by the fact the Rumanians are the only people of eastern Europe who speak a Romance language.
Romance language:  a language which has developed out of Latin.  Such languages are quite common in western Europe - for example, French, Italian, Spanish and Portuguese are all Romance languages.
The military received a major assist in pacifying the empire from the Roman policy of colonization.
Like the Phoenicians and Greeks before them, the Romans were one of the greatest colonizing people of the ancient world, planting colonies as they expanded Rome’s borders.
Comparison of Greek and Roman colonization:  A Greek colony did not maintain political ties with the mother city which had founded them. As soon as it was fully operational, it became an independent city states, politically separate from the city state which had founded it.  By contrast, Roman colonies remained politically connected to Rome.  Consequently, the modern concept of colonization, where the colony remains part of a colonial empire, comes to us from the Romans rather than from the Greeks.
Reasons for Roman colonization:
(1)  safety valve for excess population (also used by the Greeks)
(2)  to help secure newly conquered territories
(3)  to help 'romanize' the surrounding population
Military Colonies:  While many colonies were populated by civilians, especially from Italy, these were established by retired soldiers, usually close to the permanent station of the legion to which the soldiers had once belonged. Numerous throughout Rome's frontier districts, they could supply a ready reserve to the troops on active duty.
During the Pax Romana three kinds of disturbances occasionally troubled the peace of Rome:
(1)  Internal power struggles
(2)  Provincial revolts
(a)  Revolt of Britain under Queen Bodicca in the reign of Nero
(b)  The great Jewish Revolt (66-73 A. D.)
(3)  Pressure along the frontier
(a)  Germans (north)
(b)  Parthians (east)
Bodicca:  warrior queen, driven by Roman rapacity into raising Britain in revolt against the Romans during the reign of Nero.  While wiping out much of the Roman population that had come to the island, her revolt failed.
Great Jewish Revolt:  bloodiest revolt against Roman rule.  Conducted by the Jewish population, whom the Romans never liked or understood, it led to the death of several hundred thousand Jews and the destruction of Jerusalem (70 CE).
Diaspora:  a 'scattering' of the Jewish people which actually started in the last centuries BCE, but that greatly increased after the devastation of Palestine in the Great Jewish Revolt.
Despite Rome’s internal problems, some of them quite serious, the Pax Romana was the highpoint of Roman civilization.  A Roman living in this remarkable age might be forgiven for believing that the Empire in which he lived was so stable it would endure forever.  Nothing, however, lasts forever.
Marcus Aurelius:  Emperor from 161 to 180 CE, whose reign witnessed the rapid decline of the Pax Romana.  Within half a century, it seemed that the bottom had fallen out for Rome.  It is ironic that this reign should mark the beginnings of Roman decline since Marcus Aurelius ranks with the finest Rome produced.  A talented statesman, effective military leader, and writer (among Roman leaders equalled only by Julius Caesar).
Meditations:  written during his campaigns, it is one of the major surviving examples of Stoic philosophy.
“German Problem”:  In the forests of northern Europe and out on the vast grasslands to the east, the German tribes were on the move, putting unprecedented pressure on Rome's frontier began increased astronomically.