Prior to 133, Rome had expanded through various conquests. These conquests brought in additional lands, people, and the need for more soldiers. It was also during this time that the interest of the state was being usurped by self-interest within the oligarchy. The Gracchi brothers proposed reforms that would address many of the conquest issues while at the same time benefiting the state.
The oligarchy saw these as self-serving since it would help the Gracchi popularity and support. Their response was to murder them. Under Marius, a stronger Roman military was created that was more powerful and efficient. Marius also began taking power away from the Senate. Placing himself in a position where he was involved in political intrigue took away the power that he possessed. The rise of Sulla brought some power back to the Senate, but not all. Power was being siphoned away from the Senate with each new leader as they kept more and more for themselves. Sulla was the first man to be named dictator without an established time limit. Through the political maneuvering of Pompey, Sulla lost power.
A fight for power began. The formation of the First Triumvirate (Caesar, Pompey, and Crassus) gave the appearance of peace and cooperation. Instead, it was a breeding ground of discord which the Senate was all too happy to fertilize. Men were fighting over the power that would rest solely with them instead of with the Senate. The Republic was making huge advances toward being ruled completely by one man. One indication was the fact that the army became the property of its military leaders and not of the state. This changed the whole concept of power within the Republic.
The fight between Caesar and Pompey started a civil war resulting in Caesar being named dictator. The death of Caesar would open the door for more war and political schisms to form. Through the murder of Caesar, Marc Antony saw an opportunity to advance his own career. When Octavian was named the heir of Caesar, war erupted between Antony and Octavian for complete control. Peace was temporarily achieved under the Second Triumvirate, but Antony continued to pull against Octavian. Eventually, Antony formed an alliance with Egypt in order to conquer Rome and form a new empire. When Antony was dead and all rebellion suppressed, Octavian was able to focus on restoring Rome and stepping up as sole leader. Octavianís rule was the transition from Republic to Empire.
Marcel Le Glay, Jean-Louis Voisin, and Yann Le Bohec. A History of Rome. (Malden: Blackwell, 2009).