Slightly OT but in Ramayana we find such an event. When Rama wants to prepare for a war against Ravana. He asks Sugreeva(Rama's ally) to prepare the army. Sugreeva sends message to all his vassals and they all assemble at a pre-ordained time. The standing army of Sugreeva is quite small. But the assembled army is vast. Valmiki says that the army is so vast that it covers huge mountains.
Then, once Sita's whereabouts are found, Rama orders the army to march towards south in the direction of Lanka. There is an elaborate mechanism. Rama appoints a force(approx 1lakh) under a commander Neela. This force under Nala will scout ahead and find appropriate place where the army can rest. It will also be aware for any enemy movements. Rama appoints another force which will move to left flank under another commander. Rama appoints another force to move to right flank. These two forces are responsible for protecting the army's main body from any sudden attack from sides. Then, Rama appoints two experienced commanders with double the force to protect the army's movement from any swift attacks from behind. Then this huge army moves to its destination. While moving, the commanders fly in the air so as to coordinate the movement of the vast force under them. Since, managing them from ground is difficult, the commanders take to air. Sugreeva, the king, is carried in palanquin. Rama is carried by the mighty Hanuman who enlarges his form, so that Rama can oversee the movement of the entire army.
Once the army reaches the sea they decide to build a bridge across the 100yojanas of sea. Building boats to carry the vast army is time consuming and also risky because the opponents can attack the pockets of army that reach the Lanka. So, bridge is considered the best mode. Nala is an architect in Rama's army who designs the bridge and builds it with the help of soldiers. They use the timber, sand, rocks and boulders in building the bridge. Soldiers cut large number of huge trees for the required timber. Valmiki says that the surrounding area was largely deforested.
Rama's army built the bridge in 5 days and reached Lanka.
The literature gives us a glimpse into how the Indians could have handled large armies.