Gaius Julius Caesar was a Roman military and political leader that lived in the 1st millennium BC on Earth. He was instrumental in the transformation of the Roman Republic into the Roman Empire. By the 23rd century, his reputation as a dictator was on a par with Genghis Khan, Ferris, Adolf Hitler, and Maltuvis.
James T. Kirk mentioned Caesar to Roger Korby in 2266, after Korby implied Human programming leading to near-practical immortality as the major impact of transferring the Human consciousness into an android body. (TOS: "What Are Little Girls Made Of?")
The same year, upon viewing the culture on planet 892-IV, Spock noted how the culture was "an amazing example of Hodgkins' Law of Parallel Planet Development, but on this 'Earth', Rome never fell." As a result, the planet became "a world ruled by emperors who can trace their line back two thousand years to their own Julius and Augustus Caesars." (TOS: "Bread and Circuses")
Fleet Captain Garth of Izar later boasted to Kirk in 2268, that as "Master of the Universe", he would go much farther and have more success then other previous despots, such as Caesar. (TOS: "Whom Gods Destroy")
William Shakespeare's play Julius Caesar was based on the life of this dictator. Lenore Karidian, General Chang, Julian Bashir, Elim Garak and Admiral William Ross have quoted lines from this play. Garak found it strange that a "brilliant military tactician" like Caesar could not see that Brutus was trying to assassinate him. (TOS: "The Conscience of the King"; Star Trek VI: The Undiscovered Country; DS9: "The Wire", "Improbable Cause")
When Berlinghoff Rasmussen made clear that he couldn't tell Captain Jean-Luc Picard about the future, he compared the Captain's situation to that of young Caesar who might have changed his plans had he known, what lay ahead of him. (TNG: "A Matter of Time")
Julian Bashir sarcastically compared Section 31's actions to defend the United Federation of Planets to Caesar in 2375, asking "Is that what we have become? A 24th century Rome, driven by nothing other than the certainty that Caesar can do no wrong?". (DS9: "Inter Arma Enim Silent Leges")