In 60 BC, Caesar, Crassus and Pompey formed a political alliance that dominated Roman politics for several years. Their attempts to amass power as Populares were opposed by the Optimates within the Roman Senate, among them Cato the Younger with the frequent support of Cicero. Caesar's victories in the Gallic Wars, completed by 51 BC, extended Rome's territory to the English Channel and the Rhine. Caesar became the first Roman general to cross both the Channel and the Rhine, when he built a bridge across the Rhine and crossed the Channel to invade Britain.
These achievements granted him unmatched military power and threatened to eclipse the standing of Pompey, who had realigned himself with the Senate after the death of Crassus in 53 BC. With the Gallic Wars concluded, the Senate ordered Caesar to step down from his military command and return to Rome. Caesar found himself with no other options, but to cross the Rubicon with the 13th Legion, leaving his province and illegally entering Roman Italy under arms. Civil war resulted and Caesar's victory in the war put him in an unrivalled position of power and influence.
After assuming control of government, Caesar began a programme of social and governmental reforms, including the creation of the Julian calendar. He gave citizenship to many residents of far regions of the Roman Empire. He initiated land reform and support for veterans. He centralised the bureaucracy of the Republic and was eventually proclaimed "dictator in perpetuity", giving him additional authority. His populist and authoritarian reforms angered the elites, who began to conspire against him. On the Ides of March (15 March) 44 BC Caesar was assassinated by a group of rebellious senators led by Gaius Cassius Longinus, Marcus Junius Brutus and Decimus Junius Brutus. A new series of civil wars broke out and the constitutional government of the Republic was never fully restored. Caesar's adopted heir Octavian, later known as Augustus, rose to sole power after defeating his opponents in the civil war. Octavian set about solidifying his power and the era of the Roman Empire began.
Much of Caesar's life is known from his own accounts of his military campaigns and from other contemporary sources, mainly the letters and speeches of Cicero and the historical writings of Sallust. The later biographies of Caesar by Suetonius and Plutarch are also major sources. Caesar is considered by many historians to be one of the greatest military commanders in history.
Caesar was born into a patrician family, the gens Julia, which claimed descent from Iulus, son of the legendary Trojan prince Aeneas, supposedly the son of the goddess Venus. The family originated from Alba Longa, twenty miles south of Rome. The cognomen "Caesar" originated, according to Pliny the Elder, with an ancestor who was born by Caesarean section (from the Latin verb to cut, caedere, caes-). The Historia Augusta suggests three alternative explanations: that the first Caesar had a thick head of hair (Latin caesaries); that he had bright grey eyes (Latin oculis caesiis); or that he killed an elephant (caesai in Moorish) in battle. Caesar issued coins featuring images of elephants, suggesting that he favored this interpretation of his name.