Who was Julius Caesar's chief lieutenant during the Gallic Wars?

Julius Caesar's chief lieutenant during the Gallic Wars was Titus Labienus. A senior legate, Labienus was one of Caesar's most experienced and trusted commanders during the campaigns in Gaul from 58 to 50 BC.

Labienus played major roles in several operations, including the campaign against the Nervii and the fighting around Alesia. Caesar frequently mentions him in the Gallic War commentaries, often assigning him independent commands and difficult missions. His prominence went beyond that of an ordinary battlefield subordinate.

A common mix-up is to confuse Labienus with Publius Crassus, Caesar's young officer and son of Marcus Licinius Crassus. Publius commanded forces in Aquitania but was not Caesar's principal lieutenant. Labienus's later career also complicates his reputation: when Caesar crossed into civil war against Pompey in 49 BC, Labienus abandoned Caesar and joined Pompey's side. He was killed at the Battle of Munda in 45 BC.