In the aftermath of Julius Caesar's assassination in 44 BC, it was not entirely clear who would rule in the new Roman government, as well as who would take command of the armies that previously had been loyal to Caesar. It was clear, however, that two major factions would form under Mark Antony, Caesar's military second-in-command, and Octavian, Caesar's grand-nephew and heir. Octavian and Mark Antony disliked each other, but neither wanted to fight a war, especially so soon after the civil wars of Caesar.

They formed the Second Triumvirate with the help of Lepidus, who was a general himself and a chief pontiff in the Roman religion. Neither Octavian nor Mark Antony thought much of Lepidus, believing him to be loyal enough without being ambitious enough to pose a threat to them. However, this meant that the real power in the triumvirate was between Antony and Octavian, and after awhile the two men's antagonism turned into outright conflict.

Octavian defeated Mark Antony at the Battle of Actium in 31 BC. Antony famously committed suicide; his lover Cleopatra soon followed him. It wasn't difficult for Octavian (who now re-named himself Augustus Caesar) to give Lepidus a post that had authority but kept him from seizing further power. Augustus then became the first Roman emperor, holding total power.

The Second Triumvirate coalesced and then broke down because of political ambition.

The triumvirate formed after the assassination of Julius Caesar.  It formed because Octavian and the others wanted to keep power in their own hands and out of the hands of those who had conspired to kill Caesar.  Once they won that battle, however, they were too ambitious to continue to share power with one another.  Instead, Octavian and Antony pushed Lepidus out and divided the empire between them.  When that wasn't enough they fought one another, with Octavian eventually winning and becoming Augustus, the first emperor of Rome.