Yes, we can broadly reconstruct the nature of Julius Caesar’s letters to his trusted officer Aulus Hirtius using indirect evidence from contemporary sources (primarily the surviving letters of Cicero), the historical context of their close military alliance, and the surviving ancient accounts of Caesar's letter-writing habits. [1, 2, 3, 4]

While Caesar's personal correspondence with Hirtius hasn't survived directly, we know it spanned three main categories: [1, 2]

1. Political Strategy and Rapid Updates

Hirtius served as Caesar's primary political and diplomatic intermediary in Rome and abroad. [1]

• The Evidence: Cicero’s letters explicitly mention tracking Hirtius’s movements as an indicator of Caesar's intentions. For instance, right before the outbreak of the Civil War in December 50 BC, Cicero notes that Hirtius arrived late in the evening and left in the middle of the night to rejoin Caesar. [1, 2]
• The Reconstruction: Caesar wrote frequent, highly urgent letters to Hirtius to coordinate political maneuvers, gauge the loyalty of Roman senators, and handle delicate negotiations with rivals like Pompey. [1]

2. Literary Collaboration and Propaganda

Hirtius was not just a general; he was a highly literate individual entrusted with Caesar’s personal notes. He famously went on to author Book 8 of the Gallic Wars to bridge the gap in Caesar's own unfinished commentaries. [1, 2]

• The Evidence: In 45 BC, Cicero published Cato, a eulogy praising Caesar’s late republican enemy Cato the Younger. Caesar responded by writing a polemic called the Anti-Cato. Cicero’s letters reveal that Hirtius sent a preliminary copy of Caesar’s arguments to Cicero ahead of publication. [1, 2]
• The Reconstruction: Caesar sent Hirtius drafts, raw military reports, and outlines of political texts. Their correspondence involved continuous editing, collaborative speechwriting, and planning for how to distribute pro-Caesarian propaganda across Rome.

3. Highly Concise, Coded Dictations

Caesar was notorious for the rapid, concurrent dictation of his correspondence.

• The Evidence: Ancient biographers note that Caesar frequently dictated four to seven letters at once on horseback or while travelling. Furthermore, Suetonius records that for highly sensitive internal communications with his innermost circle (including Hirtius and Oppius), Caesar utilized a simple substitution cipher (the Caesar Cipher) by shifting letters of the alphabet by three positions. [1, 2, 3]
• The Reconstruction: A significant portion of the letters Hirtius received from Caesar would have been short, encrypted operational orders rather than long, reflective epistles.

If you are interested, we can look closer into how Cicero intercepted these political networks, or explore how Hirtius eventually used Caesar’s notes to write the final chapter of the Gallic Wars. Which path would you like to explore? [1, 2, 3]