How did ancient armies like the Romans, Greeks, Persians, Chinese or even at later times like the Arabs and Mongols keep the route of supplies going to support their armies marching through the vast areas they covered? Do they build supply centers in the places they conquer and use caravans to send them supplies from there? What did they do when they failed to conquer a city and they laid siege for years? What if there was not any food sources nearby?
In most cases, they would forage (or pillage) from neighboring areas. As in invading army, they had no qualms about taking whatever they wanted or needed. In situations where they were sending troops considerable distances or for prolonged periods of time, they had to develop other methods.
In some situations they would create relay stations, which would basically be a small camp where supplies could be delivered and then re-distributed to troops further along the supply line. If these stations were in areas where they were subject to attack, then they would be expanded into outposts or forts. In those cases, a contingent of troops would remain to protect the supplies that were stored up, and also possibly as armed escorts for supplies that were being passed along.