Aside from Caesar's attempt to become Emperor...

[Macilrille]

The civil war that took place when Caesar was in his late teens ended up with Sulla as dictator. He fought the war against Marius (of Marian reforms fame).

Edit: Just thought I would add, there is no consensus if Caesar ever meant to set up a permanent dictatorship along the lines of Augustus so calling it an attempt to become Emperor is misleading. He was simply a dictator, which had happened several times in Romes history, and may have eventually relinquished power.

This is true, let us not simplify history. Caesar was a traditional Roman in most ways, even if (like Sulla, Marius, the Gracchii and Scipio Africanus) he sometimes used untraditional means. Let us not ascribe him motives he might not have had.

Edited to add
Pasted from "When was Rome doomed"-thread where I originally posted it.

End of Res Publica Romana is something I have done a lot of research in, and can say for certain what caused, but remember that the end of The Republic was not the end of Rome.

Rome's constitution was made for a city state, like hundreds of others around the Med at this time. It could not cope with empire.

Problem was by and large the senate.

Manpower in Italy and thus the potential pool of recruits for the legions was dwindling, badly because the smallholders were away warring all the time. Before they had been able to war some months, then return to their farms, Cincinnatus is an example in point. As Rome gained overseas provinces it had to keep soldiers in the field year-round and they could thus not till their land. This went fallow and was to some extent taken over by magnates who tilled it using slaves. Not the Latifundia system, this was not invented yet, nor did smallholders ever disappear entirely from Italy as some ancient writers ascertain in their rethorics. For they recognised the problem as well.

One of the Scipii (I forget which) considered proposing agrarian reform in 140 bc, but was dissuaded byt his friends. Thus Tiberius Gracchus was the one to propose it in 133 bc- and pay the price. It is important to note that he proposed it as a Plebejian Tribue and to the people, just as his brother Gaius did 10 years later when he continued and even radicalised Tiberius' policy of agrarian reform and curbing senatorial power. He too paid the price, but these two had taught the people that it had power. And at this point the Plebs of Rome was numerous and volatile- it would become worse. Note that the Senate (who would loose use of Ager Publicus) resisted agrarian reforms intensely, just as they did enfranchisment (? Giving citizenship) of Italy, leading to the Social War.

Now to another, seemingly unrelated, subject. The Cursus Honorum, as Rome got more and more provinces and riches poured to Rome(Roman aristocrats) making a name for yourself- as was necessary in politics- became more and more expensive. Building projects, Gladiatorial games and free grain became a necessity. To name an example Caesar was deep in dept to Crassus from this. This means that the aristocrats greed became larger, they needed money if they were to make a name for themselves, and they could only pay back those debts by propraetorship or proconsulship, which would allow them to skim the incomes from the province. Even honest men were caught in this trap, for all of them had generations of great men and expectations on their shoulders, they HAD to climb Cursus Honorum and do great things. Competetion thus became more and more intense and ruthless, end more and more expencive. Catilinia was a point in case, he failed and was so indebted that he had basically no other choice then try a coup. Now, remember this if you please.

Next step towards destruction was taken by Marius, he did not in fact professionalise the army as has been often ascertained, the average service time remained 6-7 years as it had been through all 2nd century BC. What he did was enroll everyone without considering the limits on income. Others had in fact done this to some extent, but he got a massive wave of volunteers who suddenly saw prospects for land when service was over. Rural Plebs, not urban, made up Marius' new army and indeed it was loyal only to him.

This brings us to good old Sulla, senate gave him command against Mithidrates of Pontus, and he wanted it cause Asia was very rich- much loot- People gave command to Marius, so Sulla used his army, made on the new model and loyal only to him, to march on Rome itself!!! and enforce the Senate's decision.
Marians took power while he was gone and repressed his followers and he exacted bloody revenge when he returned, with HIS army- loyal only to him, gained dictatorship, whith his army, gave them land and reformed some laws, etc. he then resigned and died.

The one to learn all these lessons, about the power of the people and the power of a private army was Gaius Julius Caesar, intelligent and ambitious, he used all the lessons learned by looking at Gracchii, marius, Sulla, and he gained absolute power. The Republic was dead.

But what killed it?

As should be evident, the depletion of recruits caused by the Senate's reluctance to agrarian reform and enfranchisment of Italy led to the recruiting of private armies that were loyal only to their general as only he could reward them sufficiently with land. This was one "branch of the cause".

Ambitious patricians had to spend more and more as the competetion in Cursus Honorum and provincial commands grew more intense. At the end people like Sulla and Caesar were willing to do anything, genocide, turning on Rome itself, proscriptions in Rome... to gain power, fame and a name. The Senate's stubborn resistance to reforms handed these ambitious men the ultimate tool, private armies, and with them, they killed Res Publica Romana.

All of this, recruitment for legions and the Cursus Honorum, as well as the way conquered land was shared was part of the constitution of Rome, the CITY STATE constitution that could not cope with empire and the riches it brought while taking the soldiers from the land.

You will notice that Caeasar mostly did what others had and followed Cursus Honorum, only by becoming Dictator for more than half a year did he deviate more than his predecessors.

Hope that helped.
In case you wish to learn more and in more detail, here is some literature on the matter.

Badian, E.: Roman Imperialism in The Late Republic, Oxford, 1968.
Beard, Mary & Crawford, Michael: Rome in The Late Republic, London, 1999.
(a) Brunt, Peter: Social Conflicts in the Roman Republic, London, 1971.
(b): Italian Manpower, Oxford, 1971,
Harris, W. V.: War and Imperialism in Republican Rome, 327- 70 BC, Oxford, 1979.