After the assassination of Julius Caesar, there was a power vacuum in Rome. Unlike previous leaders, Caesar had become enormously powerful as a dictator, and when he died nobody was sure what would happen next. Would there be a new sole dictator? Would they return to the ways of the Roman Republic?
After a series of military battles, Rome settled on a very interesting political structure to answer that question. For ten years, from 43 B.C.E. to 33 B.C.E., they were ruled by a Triumvirate, an alliance of three men who split up the duties of governing. Marc Antony, Octavian, and Lepidus were each rulers over separate portions of the Roman Empire, and shared rule over Italy and Rome itself.
Origins of the Triumvirate
The three men who formed the Triumvirate were powerful political leaders before the assassination of Julius Caesar. Octavian was Caesar's adopted son and heir, and had a strong claim to his title. Marc Antony was a general who had served under Caesar in his wars in Gaul and had enormous political influence. Lepidus was an influential political and military leader who had control of a large army near Rome.
In the immediate aftermath of Caesar's assassination, Antony held a lot of political power in the Republic. The Senate wanted him to return the Republic to the state it was before Caesar took over, abolishing dictators and restoring the Senate's power. Although he took some steps to make that happen, he eventually started to use Caesar's image for political gain. Caesar was very popular with the Roman masses, and Antony sensed he could use that popularity to gain power for himself and turn the people against the Senators who had killed Caesar.
This lead to a series of military battles between Senate forces and troops that were loyal to Caesar and Antony. After a few evenly-matched battles, Antony retreated outside of Rome and the Senators who had organized the assassination fled Rome. Lepidus, who was waiting with his troops outside of Rome, stepped in and began negotiating with Antony. They organized the alliance that would eventually include Octavian and become the Triumvirate.
Going into the negotiations, Octavian had a rightful claim to power but no military forces to enforce his claim. Lepidus had many legions of troops, but no real claim to power, and Antony had some claim to power and a fair amount of troops, although they had just recently lost a battle.
When they formed the Triumvirate, they agreed to each take some land to rule for their own: Lepidus took Hispania (Spain), Antony took Gaul (France) and Octavian took North Africa. They agreed to joint rule over the remaining territory. Crucially, Lepidus gave control over many of his own troops to Octavian. In 43 B.C.E., one year after Caesar's death, the Triumvirate was established by law
Fall of Lepidus
Now that the Triumvirate was established, the leaders started on their next political goal, killing the Senators who had organized the assassination of Caesar. Antony and Octavian were tasked with killing the Senators Brutus and Cassius, who had fled to Asia Minor. They raised new legions, and the two Triumvirs defeated the Senators in the famous Battle of Philippi, where Brutus and Cassius killed themselves rather than be captured.
The defeat of these two Senators brought new land into the Roman empire, and the three Triumvirs met to reallocate the new provinces. Because Antony and Octavian lead the forces that conquered the land, they ended up receiving the bulk of the new land. Lepidus was left with very little for himself, and no longer had a military advantage over the other two. Thus, his power in the alliance began to fade.
It is important to note that we cannot ever be sure of what the exact balance of the alliance was, because it is so far in the past. Ancient historians did not have the same standards for historical writing that we do, so simply reading an account from around the time the events occurred can be very misleading. Nowadays, historians can use roundabout ways to try and determine the balance of power in the alliance and make arguments to support their conclusions. Ancient leaders were fond of putting their faces on coins, so if historians discover many coins with Antony or Octavian's face on them, but few or none with Lepidus' face, they argue that Lepidus was losing power.
In any case, the events leading to Lepidus' downfall followed after the Battle of Phillipi. After the territories had changed hands, Lepidus attempted to seize control of Sicily by stationing many of his troops on the island. Octavian countered that after the reallocation, Sicily was his. In a stinging defeat for Lepidus, his own troops sided with Octavian and defected. Left without little army and on the outs politically, Lepidus was forced into exile and the Triumvirate was down to two.
The Final War
With the Triumvirate falling apart, it became clear that Antony and Octavian were inevitably going to collide. In order to win favor with the Roman people, both fought wars that conquered new territory and won riches for the treasury. Antony was beloved by the people, and Octavian looked for ways to undermine that popularity.
To cement the alliance, Antony had originally married Octavian's sister, Octavia. Octavian discovered that, rather than living with his wife, Antony was living with the Egyptian leader Cleopatra. He knew he could use this to his advantage, because the Roman people were very suspicious of foreigners, and would look down on one of their leaders rejecting his Roman wife and preferring an outsider. To top it off, Octavian found out that not only had Antony had children with Cleopatra, but he had given them preferential treatment over his Roman children in his will. Octavian revealed all of this to the Roman people, and a final battle between the last two Triumvirs became inevitable.
The war, often called the Final War of the Roman Republic (because afterwards it was no longer a Republic but an Empire), was declared in 32 B.C.E. In response to the discoveries in Antony's will, the Roman Senate declared war on Cleopatra and Egypt. Octavian orchestrated this, knowing that Antony would come to her aid and he would get his decisive civil war.
Because Egypt and Rome are separated by the sea, much of the fighting was in a large naval battle. After much manoeuvring in the open ocean over the course of a year, Octavian defeated Antony in the Battle of Actium, using his superior numbers and more experienced soldiers to defeat the combined navies of Antony and Cleopatra. Actium was the end of the Triumvirate. Antony retreated to Alexandria where he and Cleopatra died shortly after. Octavian alone was the ruler of Rome.
In honour of his victory, Octavian was given the name Augustus, which he embraced along with his title as son of Caesar. With no rivals, and holding much of Rome's military power and almost all its land, Augustus was an unstoppable force in the Rome. He quickly got the Senate to pass a series of laws granting him the title of sole Emperor.