Claim: Julius Caesar crossed the Rubicon in the 40s BC.

Gaius Julius Caesar -LRB- -LSB- ˈɡaː.i.ʊs ˈjuː.li.ʊs ˈkae̯.sar -RSB- ; 13 July 100 BC -- 15 March 44 BC -RRB- , known as Julius Caesar , was a Roman politician , general , and notable author of Latin prose . He played a critical role in the events that led to the demise of the Roman Republic and the rise of the Roman Empire .   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 refused the order , and instead marked his defiance in 49 BC by crossing 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 centralised the bureaucracy of the Republic and was eventually proclaimed `` dictator in perpetuity '' , giving him additional authority . But the underlying political conflicts had not been resolved , and on the Ides of March -LRB- 15 March -RRB- 44 BC , Caesar was assassinated by a group of rebellious senators led by Marcus 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 .