After negotiations collapsed in early January, the Senate used a senatus consultum ultimum against Caesar, signaling that force could be used to defend the state and helping precipitate open conflict.
Caesar crossed the Rubicon with his army, illegally entering Roman territory under arms and initiating the civil war between Caesarian forces and the Pompeian-led senatorial faction.
Caesar’s forces besieged Corfinium and compelled the surrender of Domitius Ahenobarbus’s garrison, an early and politically significant success that accelerated Pompeian withdrawals from Italy.
Massilia (Marseille) resisted Caesar and aligned with Pompeian interests; Caesar left subordinates to conduct a prolonged siege and naval blockade while he pursued operations elsewhere.
In the Ilerda campaign, Caesar neutralized major Pompeian forces in Hispania through maneuver and pressure, removing a western threat before turning east toward Pompey’s main army.
After months of siege and naval fighting, Massilia capitulated to Caesarian forces, ending a significant western Mediterranean holdout tied to Pompeian resistance.
With Italy secured and the Spanish campaign concluded, Caesar was appointed dictator in Rome, giving him extraordinary authority to manage elections and state affairs during the emergency.
In the hard-fought operations around Dyrrhachium, Pompey gained an important tactical victory over Caesar, briefly checking Caesarian momentum before the decisive battle in Thessaly.
At Pharsalus, Caesar decisively defeated Pompey’s army, prompting Pompey’s flight and triggering major defections in the eastern provinces to Caesar’s side.
After fleeing Pharsalus, Pompey reached Egypt but was murdered on the orders of King Ptolemy XIII, eliminating Caesar’s chief rival while pulling Caesar into Egyptian dynastic conflict.
Caesar became embroiled in the Alexandrian War and endured a siege in Alexandria (late 48–47 BCE), as fighting intensified between Roman forces and factions supporting Ptolemy XIII versus Cleopatra.
In early 47 BCE, Caesar and allied forces defeated Ptolemy XIII’s army near the Nile Delta, securing Cleopatra’s position and allowing Caesar to conclude major fighting in Egypt.
Caesar rapidly crushed Pharnaces II near Zela in Asia Minor, a swift victory later associated with his report veni, vidi, vici, before he resumed dealing with remaining civil-war opposition.
At Thapsus in North Africa, Caesar defeated a major Pompeian coalition led by Metellus Scipio and allies, effectively collapsing organized senatorial resistance in Africa.
At Munda in southern Hispania, Caesar won the final major battle of the civil war against the Pompeian leadership, ending large-scale organized resistance and consolidating his dominance.
Caesar's Civil War: Crossing the Rubicon to Munda (49–45 BCE)
