So basically, I'm in a college hisory class right now, and I'm having trouble piecing together all the details I hear thrown around about the rise of Rome as a Republic and ultimately its transition into an Empire. I get that Ceaser played a large role in the decline of the Reublic, essentially becoming Rome's dictator, but it seems he was more of the final straw rather than the main cause. What strengths of the Republic that allowed it to last so long disappeared as it transitioned to an empire? Like I said, I've heard tons of theories as to why the Republic failed, like latifunium, expansion (although I'm hazy on specifics here), etc., but I'm having a hard time seeing how they all relate to each other and caused Rome to transition from a Republic to an empire.
Firstly can I just frame your question a little. We need to be careful about what the "transition to empire" actually entailed for us to understand the causes. The Senate, Consuls and popular votes all remained in place during the Principate. So what changed?
one man commanding entire military apparatus from Rome (holding proconsular power over those regions as a result).
one man possessing a collection of magistrate powers that allowed him to present legislation, make appointments and veto.
Hereditary holding of this supreme magistrate office.
Essential this change is born out of unrest. Rome has a really good political system that mixes oligarchy with popular votes that in the 2nd Century is praised by the Greeks. However this system is perfect for running a city and not the vast Mediterranean empire she aquires. It has flaws throughout it such as the need for magistrates to physically perform their duties in person. When Sulla left to take command of his army a crony of Marius set up a popular vote that removed him from command of the army. Sulla could not stop this as he wasn't physically in Rome at the time. It also meant that if you wanted to stop a tribune or Consul vetoing a motion you just stop him getting to the assembly.
The small town elite rivalries of the city were being played out with the resources of the Mediterranean. Nobles would go to a province in Spain, screw as much gold out of the population as possible, start a fight with a tribe to win a military reputation then go back to Rome to aggrandize themselves and lay low their rivals. As the rivalries became more intense provinces were being bled dry.
This rivalry broke into civil war on a number of occasions, Sulla & Marius, Caesar & Pompey being the big ones. The changes made by Augustus were successful steps to remove the cause of the rivalries (power & soldiers).
How does Caesar fit into all of this? Whilst we can agree the root causes of the civil strife that lead to the rivalries was the strain of ruling an Empire with a city state apparatus, there is the question of timing. Rome gets its large Empire early but it takes quite a long time for the issues to result in strife. This is because the Romans, nobility in particular, have a strong sense of what they called "The way of the ancestors". This was their perception of how their forefathers acted and a strong need to always act the same way despite it not being strictly mandated by the law to do so. For example the Senate had no legislative power but was a useful advisory body, proposals were usually presented there for comment and only taken to public vote if the senators were happy with it. Legally all that mattered was the public vote. There were many unwritten rules like this.
Rivalries still emerged but no politician stepped over these perceived ways of doing things and so the Republic still functioned. However this begins to change with the Grachii brothers who adopt a more popular political stance and begin to adopt tactics that broke the unwritten "ways of the ancestors" without breaking the law. To combat them the Senate breaks the ultimate break and has the brothers killed. Examples have been set and two groups of nobles emerge. Ones that cling onto the old ways (Optimates) and ones that are willing to break them to climb the ladder (Populares). Caesar is the last of these men, many unwritten rules have been broken by his turn and he breaks them again and then the rest. He has himself declared Dictator for Life with no reason (dictators would always need a reason such as "winning the war against Hannibal" so they could retire when the job is done) among other things. This isn't some hell bent rampage to wreck Rome, he knows it's broken and takes many steps to try to fix it such as the Lex Julia reigning in the corruption in the provinces. But overthrowing the republican government and setting himself up as dictator for life is the final nail. His achievements also set up Augustus with a massive client base and heft of power with which to eventually make himself master and enact those changes. Tellingly Augustus framed his changes as bringing back the values of the past.