Before the birth in July 100 B. C. of Julius Caesar and his subsequent career, which changed Rome dramatically, the Roman Empire (probably dating from about 30 B. C.) extended its absolute control through conquest and political alliances of not only most of Italy but also most of the countries and territories bordering the Mediterranean--southern France, Spain, North Africa. This extended rule is all the more remarkable because the central authority was centered in the city of Rome itself. Rule before the advent of Caesar's control consisted of two consuls from the wealthiest class and the senate, which was also made up of the wealthiest citizens of Rome. In effect, Rome and most of the Mediterranean countries were ruled by an oligarchy, a small group consisting of one class, in this case, the wealthy and politically connected. Rome at this time, however, is a republic.
Julius Caesar, whose political life began in the late 50s B. C. and blossomed in the decade of the 60s B. C., became the single most important transitional figure in Roman history. In the early 60s, Caesar, whose goal was consulship of Rome, allied himself with Pompey and Crassus, and these three created what is known as the First Triumvirate. Although unofficial, the First Triumvirate ruled Rome with the consent, if not the heart, of the senate.
Ostensibly, the three consuls held roughly equal powers, but during this period, Caesar became Rome's most successful military leader. Caesar took his Legions into northern Gaul (essentially, northern Italy and southern France) and conquered the indigenous people, who had been a military threat to Rome for many decades. Over the next several years (in the 50s B. C.), Caesar and his Legions managed to conquer all of Gaul (most of modern France), victories that made Caesar not only popular but more politically powerful than his fellow consuls, Pompey and Crassus, in Rome.
After the death of Crassus in 53 B. C., Caesar and Pompey, still consuls, gradually became enemies over the struggle of who deserves to be the primary power-wielder. When Caesar was still in Gaul, and he and Pompey had declared unofficial war on each other, and Caesar took his most famous step--he and his Legions marched back into Roman territory without permission of the senate by crossing the river Rubicon. We now use the phrase "crossing the Rubicon" to describe an irrevocable step.
Caesar became, after threatening Rome with his Legions and convincing the senate that they were at his mercy, the first Roman dictator (actually, Emperor), at first a temporary title, but made permanent in 44 B. C. Rome's transition from republic to dictatorship came to a sudden conclusion with the ascendance of Caesar to Dictator, and Rome became the Roman Empire for the rest of its life as a political and military power. The senate still existed, but its power devolved onto the man designated Dictator.
Although we cannot characterize Caesar's reign as Dictator as a benign (harmless) dictatorship, his reign is characterized by forward-thinking programs: he essentially gave amnesty to many of his political and military enemies; he succeeded, at least temporarily, in making the senate more representative of the people; honoring promises to his military forces by giving them land for their retirement from active duty; and attempted to restructure Roman rule in conquered territories to provide more autonomy.
Caesar, who did not have a natural heir, adopted his great nephew, Octavian, as his heir. During the decade of the 40s, opposition to Caesar's dictatorship grew in the senate and, on March 15, 44 B. C. (now memorialized as the "Ides of March"), Caesar was assassinated by a group of senators, an action partly brought on by his involvement with Cleopatra in Egypt, but largely because the Senate was tired of Caesar.
Octavian, who became the Emperor Augustus, is considered the first true Emperor, as opposed to Dictator, and the reign of Augustus is often considered the beginning of the Roman Empire. From a political standpoint, however, Caesar's reign moved Rome from a republic to an empire whether we call Caesar a dictator or an emperor. From about 44 B. C. to its end in 476 A. D. (the "western' Roman Empire), the Roman Empire wielded unprecedented powers over a wide swath of the Mediterranean world and, along with Greece, sowed the seeds of modern civilization.