I've heard from an older post on here that Caesar reached Iberia from Rome in less than 30 days of marching, travelling some 1,500 miles.
Inheritance of Rome by Chris Wickham states on page 26 that in the late Empire, "to travel from Rome to northern Gaul took a minimum of three weeks, an army would take much longer". Julius Caesar, in the late Republic, was able to march 1,500 miles/2,400 km from Rome to Obulco (in Spain) in less than one month, which was and is considered an impressive feat.
I'm very curious about this impressive march, if anyone has more details on it. In particular, though, I wondered how baggage and supplies were handled during it.
For example: Do we know if Caesar took supply wagons with him from Rome, all the way to Iberia, and that they kept up with his impressive pace? Or is there some other method of resupply he arranged which we know about?
So for this particular trip, we simply do not know much about it, other than Caesar makes good time, and wrote a poem to brag about it, Iter, or "Journey" (Suet. Caesar. 56, and the exact duration of the trip is disputed) Its unclear how much was by ship or by land (presumably at least part was by sea, especially for his army). That said, in general a key aspect of Caesar's logistics was requisitions from communities in the area of operations. Indeed, one of the clues that you have been conquered by Caesar in Gaul during the 50s BC was the fact that he demanded grain from you to feed his army, both to support operations and also in garrison. During the civil wars, providing money and supplies to an army was a mark of either loyalty or fear---failure to do so would be a clear sign you had picked the other side (for example Massilia, in gathering in its wheat stockpiles, not only prepared for a siege, but denied these resources to Caesar (BCiv 1.34)
Roman soldiers did forage for grain, but the less time foraging the better: troops foraging were vulnerable to attack, and every day spent foraging was a day that was not spent in movement or maneuver. The brisk pace of Caesar's march suggests suggests virtually no foraging, but that supplies were provided along the way.
Now by the time of Caesar's Iter to Spain, his military and political position was such that he likely had unfettered access to the tithe grain of Sicily and Sardinia, which had been used to supply Roman armies during the Middle Republic. However, over the course of the late Republic, a great deal of this grain was diverted from military supply to the grain dole in Rome, and Caesar's motive for reducing the number of dole recipients, supposedly from 320,000 to 150,000 (Suet. Caes. 41.3 , hardly a popularis act!), may have been intended to free up grain for military use. Still, even by the Munda campaign most grain is likely being requisitioned locally, requiring Caesar and his agents to make constant new arrangements with the communities they passed through.
The two key works of Roman military logistics remain P. Erdkamp, Hunger and the Sword and J. Roth, The Logistics of the Roman Army at War.