Cicero to Plancus, written from Rome around the end of May 43 BC — Perseus dateline Scr. Romae circ. ex. m. Mai. a. 711 (43). The reply to a dispatch of Plancus’s (10.15 or a near companion) that reached the Senate in a packed house. The scene is set with characteristic Ciceronian relish: the urban praetor Marcus Caecilius Cornutus, presiding in the consuls’ absence, had just finished reading out a tepid letter from Lepidus when Plancus’s was put before him. The contrast was the news.

What Cicero is telling Plancus, under the report of the proceedings, is the substance of the second section: ipse tibi sis senatus — be your own senate. The phrase is famous, and it is the boldest political instruction Cicero ever issued to a serving consular commander. He is telling Plancus that with Lepidus wavering and Brutus and Cassius still in the East, the state has no machinery left quick enough for the crisis; the field commander on the Allobrogan frontier must decide on his own authority, and the Senate will ratify what he does. Cicero’s promise is the guarantee of senatorial cover for whatever Plancus dares. Within a fortnight Lepidus’s army would defect and the Plancus–Lepidus front would collapse; this letter belongs to the brief window of optimism between Mutina and that collapse.