M. Caelius Rufus to Cicero, written from Intimilium on the Ligurian coast on the sixteenth day before the Kalends of May (16 April) 49 BC (manuscript dateline Scr. Intimili xvi K. Mai. a. 705 (49)). This is the famous defection letter — one of the most-cited pieces of evidence in the entire correspondence for how rapidly the political class of the late Republic realigned in the spring of 49. Caelius is in Caesar’s camp, on the march up into Liguria and over the Alps en route to the Spanish campaign that will decide the war in the West. Cicero, whom Caelius last saw still hesitating, has sent a letter that conveyed, without saying so outright, that he had resolved to cross over to Pompey. Caelius writes back the same day, “stunned,” to talk him out of it.

The voice is the Caelian voice raised in alarm: breezy when he needs to be (the gibe in section 2 that Cicero is “ashamed of being too little of an optimate”), brutally direct when it counts (Caesar “thinks — and even speaks — of nothing but harshness and savagery”). Caelius works every argument he has: family (filius unicus, the son-in-law Dolabella, the house, the hopes); the political logic of the moment (Caesar already takes Cicero’s hesitation as offence; to act now is to join the routed party after refusing to follow them when they stood firm); the strategic forecast (once Spain falls, what can the Pompeians offer?); and his own dignity (had Caesar not been bringing him to Spain, Caelius says, he would have ridden to Cicero himself and held him back “with all my force”). The last section comes down to the compromise he wants Cicero to accept — not a defection to Caesar, but withdrawal to some small town free of the war, which is the position Cicero will in fact occupy for the rest of the year, and which will protect him after Pharsalus. The letter is also a striking piece of contemporary testimony on the limits of Caesar’s much-advertised clemency: the Corfinium pardons did not, in Caelius’s hearing, foretell what Caesar meant to do after a final victory. “He thinks — and even speaks — of nothing but harshness and savagery.”