Although it was not clear at the time, Caesar’s consulship marked the end of the Roman republic. From the date of his accession in 59 BCE, the traditional forms of government had become untenable, and his reign directly led to the civil war that followed a decade after his death. Caesar began his career primarily as a military general, and he expanded the area of Gaul that the Romans possessed sometime in the 60s BCE. He had a reputation for brutally putting down revolts of poorly-organized German tribesmen, and he may have played a role in quelling the ongoing rebellion in Parthia. His military acumen had made him perfectly poised to take his position in the Senate by force.

Around 59 BCE, Caesar, allied with Pompey and Crassus in a league known as the “First Triumvirate,” took power, much to the dismay and disapproval of the Senators. In particular, Caesar’s long-time rival, M. Porcius Cato, believed that Caesar’s illegitimate rise to office threatened the integrity of the election procedure and law-making ability of the Senate, and he staunchly opposed Caesar’s rise for the rest of his life. While in office, Caesar passed legislation that many of the Senators considered invalid, leading them to hatch a plot in which they would separate the support of Pompey and Crassus from him and allow them to exercise a dual consulship. It should be noted that none of this marked the traditional politics of Roman antiquity, and the mores and habits of the Senate were falling apart.

By 47 BCE., Caesar had returned to the capital after campaigning in Gaul and successfully overthrowing the dual consulship of Crassus and Pompey. Much of this had to do with Caesar’s self-created image, in which he paraded the wealth and glory he had attained as a result of his defeat of the Gauls for all Roman citizens to see. This tremendously increased his popular support from the people and made the Senators hate and fear him. Worse yet, upon his return, Caesar declared himself dictator for life, a condition of rule no one was willing to accept. The Senate had determined that the only way to wrest power away from Caesar was assassination. In 44 BCE, M. Junius Brutus, Cato’s nephew, and C. Cassius Longinus killed Caesar on the steps of the Senate building.

Christopher S. Mackay, Ancient Rome: A Military and Political History

Julius Caesar is one of the most famous political and military figures in world history. Without him and his populist leanings and activities the Roman Senate might never have opposed him as vehemently, and thus would have not changed to the extent it did.

Caesar grew to manhood during the time of The Roman Republic, not The Roman Empire. His life and contributions would cause his heir, Augustus, to be the first true "emperor".

His relationship with the Senate was always one of conflict and uneasy tolerance. The disregard that Caesar showed for the wishes of the Senate in his military days are well summed up by his crossing the Rubicon in 49 BC in defiance of their orders. Hence the term "crossing the Rubicon" has come to mean taking an action that puts one across the point of no return.

The Life and Times of Julius Caesar, by Whiting

Caesar: A History of the Art of War Among The Romans, by Dodge