Lucius Munatius Plancus ( – ) was a Roman senator, consul in 42 BC, and censor in 22 BC with Paullus Aemilius Lepidus. He is one of the classic historical examples of men who have managed to survive very dangerous circumstances by constantly shifting their allegiances. Beginning his career under Julius Caesar, he allied with his assassin Decimus Junius Brutus in 44 BC, then with the Second Triumvirate in 43 BC, joining Mark Antony in 40 BC, and deserting him for Octavian in 32 BC. He also founded the cities of Augst and Lyon. is still visible at Gaeta.
Early career
Plancus was born in Tibur, the son of his homonymous father, of whom very little is known. He had three brothers and a sister: two of the brothers pursued public lives, one ascending to the praetorship and the other reaching the plebeian tribunate. He must have entered public life with election as quaestor some time before 54 BC.
More concrete information on Plancus' career only appears when he became one of Julius Caesar's legates during the Gallic Wars: he served under Caesar in Gaul from 54 BC through to the start of Caesar's civil war in January 49 BC. At the start of the civil war, he joined with Caesar against Pompey; he served under Caesar with Gaius Fabius (the praetor in 58 BC) during Caesar's Illerda campaign. He sailed with Caesar to Africa in 47 BC; this year he likely was one of the praetors. When Caesar left for Spain in 45 BC, Plancus was appointed one of the prefects of the city in place of quaestors and aediles who had not been elected that year. Upon Caesar's return from Spain, he appointed Plancus governor of Transalpine Gaul pro consule in early 44 BC.
Transition to empire
Upon the assassination of Julius Caesar on 15 March 44 BC, Plancus was still in Rome. In the immediate aftermath of the deed, he – like many other Caesarians – supported amnesty for the tyrannicides and departed for his province. He must have also supported preservation of Caesar's acta, which would have been in service to his own career as well, as Caesar had before his death designated Plancus as one of the consuls for 42 BC. There, he raised more men while keeping a close eye on political developments in the city. He also campaigned in Raetia, winning some victories, for which he was proclaimed imperator, by September.
Formation of the triumvirate
A political crisis unfolded over the year 44 BC. Plancus and Decimus Junius Brutus were the governors of Transalpine and Cisalpine Gaul, respectively. Mark Antony, then one of the consuls, used soldiers to intimidate the assembly in passing a law in June transferring those provinces to himself. Antony left on 28 November 44 BC seeking to take those provinces from their governors. Soon afterwards, Cicero was writing to the governors already stationed there seeking military support to resist Antony's assumption of the provinces; he wrote both to Plancus and Decimus Brutus. On 20 December 44 BC, the senate decreed that the existing governors should resist Antony by force if necessary. His funerary inscription attests that around this time he founded the cities of Augusta Raurica (44 BC) and Lugdunum (Lyon) (43 BC) and in June 43 BC, some letters attest to his passage through the village of Cularo (present Grenoble) in the Dauphiné Alps.
Plancus was slow to respond to the senate's call to arms. Responding months later in March 43 BC, he publicly warned against the rush to war. Privately, he wrote to Cicero that he was preparing his legions and auxiliaries. While both sides were preparing forces, it is clear that Plancus was entertaining offers from both Cicero in the senate and Antony. Around the same time a further message arrived from Plancus publicly declaring for the senate, Antony asserted in a letter to Rome that he had Plancus' support. Plancus defended his caution with the argument that declaring too rashly before preparations would have meant swift consequences like those that befell Decimus Brutus (then besieged in Mutina).