The Romans were supremely organised and disciplined, with a well-functioning supply system utilising both private and public sources. They were also experienced: some men made careers out of logistics, and officials such as head of the grain supply were powerful and respected. In addition, they had the infrastructure in place, with things like warehouses and roads, as well as the engineers to build and maintain them.

They had good communication networks, such as the famous cursus publicus. You have to have good information on where the supplies are and where they are needed - as well as the competence to predict the future, and estimate where supplies will come from and where they will be needed. We can’t forget they also had the security apparatus in place to protect roads, sea routes and supply chains. Finally, they had the money necessary to invest in such a system.

Sources: Peter Temin, who has done excellent work with the Roman economy, and Jonathan Roth’s The Logistics of the Roman Army at War.