The Battle of Pharsalus was a decisive battle of Caesar's Civil War

Caesar's civil war

The Great Roman Civil War , also known as Caesar's Civil War, was one of the last politico-military conflicts in the Roman Republic before the establishment of the Roman Empire...

. On 9 August 48 BC at Pharsalus in central Greece, Gaius Julius Caesar

Julius Caesar

Gaius Julius Caesar was a Roman general and statesman and a distinguished writer of Latin prose. He played a critical role in the gradual transformation of the Roman Republic into the Roman Empire....

 and his allies formed up opposite the army of the republic

Roman Republic

The Roman Republic was the period of the ancient Roman civilization where the government operated as a republic. It began with the overthrow of the Roman monarchy, traditionally dated around 508 BC, and its replacement by a government headed by two consuls, elected annually by the citizens and...

 under the command of Gnaeus Pompeius Magnus

Pompey

Gnaeus Pompeius Magnus, also known as Pompey or Pompey the Great , was a military and political leader of the late Roman Republic...

 ("Pompey the Great"). Pompey had the backing of a majority of senators

Roman Senate

The Senate of the Roman Republic was a political institution in the ancient Roman Republic, however, it was not an elected body, but one whose members were appointed by the consuls, and later by the censors. After a magistrate served his term in office, it usually was followed with automatic...

, of whom many were optimates, and his army significantly outnumbered the veteran Caesarian legions.
A dispute between Caesar and the Senate of Rome culminated in Caesar marching his army on Rome and forcing Pompey, accompanied by much of the Roman Senate

Roman Senate

The Senate of the Roman Republic was a political institution in the ancient Roman Republic, however, it was not an elected body, but one whose members were appointed by the consuls, and later by the censors. After a magistrate served his term in office, it usually was followed with automatic...

, to flee Italy to Greece in 49 BC where he could better conscript an army to face his former ally.