Julius Caesar had joined with Pompey and Crassus in 60 BCE to form a political alliance known as the First Triumvirate, in an attempt to put pressure on the Senate to pass bills in their favour. The following year he was elected Consul and began to push laws through with the support of his political allies.

After his consulship ended in 58 BCE, Caesar was able to secure for himself the governorship of a number of provinces that brought at first four legions of troops under his control. By 50 BCE he had conquered Gaul, but the First Triumvirate had collapsed. In Rome, the Senate had made Pompey sole consul and ordered Caesar to disband his army and return to Rome as a regular citizen.

Concerned that he would be prosecuted for historic crimes from his time as consul, Caesar instead chose to make his way to Rome with the 13th Legion. The small Rubicon River marked the border between Cisalpine Gaul and Italy, and crossing this with an army was a capital offence. Caesar knew that doing so would be seen as an act of war, and the phrase ‘to cross the Rubicon’ is now used to refer to actions that commit a person to a specific course.

In Caesar’s case he crossed the Rubicon on 10 January 49 BCE, though the exact circumstances are widely debated. The Roman historian Suetonius later claimed that Caesar deliberated until a supernatural apparition prompted him to cross. Sources disagree over what he said next, though all versions involve a die being cast.

As Caesar approached Rome, Pompey and the Senate fled south. The subsequent civil war lasted for over four years, ending with Caesar’s victory and him being declared dictator for life.