Caesar wrote two very important accounts (commentarii), the first about his wars in Gaul (de bello Gallico) and the second about the ensuing civil war between himself and Pompey (bellum civile).

BG is invaluable as a source for military historians of the Late Republican army. He frequently describes a myriad of different aspects of Roman military practice such as the construction of camps, bridges, and siegeworks, the organization of forces in legions and cohorts, equipment, division of power, et cetera. BG is also, unfortunately, the primary literary source for information on the victims of his conquests, the La Tène culture Celts (Gauls) of western Europe in the mid-1st century BCE. In Book VI of BG, Caesar gives us a little miniature ethnography of the Gallic/Belgic/Germanic tribes. It is amateur in the extreme, but it is something, and there is no better alternative as far as written sources. The BG is important for many other reasons, but let me mention one more: it is an important example of Latin style and syntax in this period, to be compared to other long-form prose works like Cicero's speeches. It is also the ONLY example we have in the genre of Roman commentarius, although we understand that Caesar's works should not be taken as typical. They are literary commentarii.