Modern generals are granted access to a multitude of communication and information gathering technologies from satellite imaging to UAVs and infantry equipped with cameras. Ancient generals had none of these luxuries, and even methods of communication such as flag signaling were rare or nonexistent among some ancient cultures.
In large scale, ancient battles generals had to orchestrate the movements of tens of thousands of troops over thousands of acres of ground. Getting a wing of an army to change its course involved sending a rider. Often as battles wore on they created clouds of dust which obscured a commander’s view and night operations were incredibly difficult to manage as well.
To negate the difficulties of commanding thousands of troops at once, many generals organized a previously agreed upon and practiced deployment and attack. The Romans and many other cultures organized battle marches that led the army across the battlefield in three lines before turning towards the enemy and presenting an ordered battle formation.
Part of the reason the phalanx was so popular is that it required very little micromanagement during battle, this was the same for the Roman manipular formation, though movements such as when and how to deploy the Triarii were decided by the commander during battle.
With deploying and managing an army of thousands being such a difficult task, many generals stood out from the crowd because their leadership and grasp of command led to scores of victories. Generals such as Alexander the Great, Hanibal Barca, Scipio Africanus, and Julius Caesar are well known as the best ancient commanders, but how exactly did they command and influence the outcome in battles involving tens or hundreds of thousands of troops?
As mentioned above, it was very common to ensure that pre-battle formations were set up correctly, often with the best infantry in the center or the right flank (traditionally the general set up near his right flank) and cavalry often split equally on the wings. Great commanders could exploit this setup by changing small aspects of their formation to achieve victory.
An excellent example of this is Caesar’s formation at Pharsalus. Facing an army with nearly twice the infantry and about five times more cavalry, Caesar knew he couldn’t win in a straight head on fight. Taking selected men from the traditional three lines, Caesar formed a small and concealed fourth line stationed on the right. The fourth line of infantry charged into the cavalry fight and helped Caesar’s 1-2,000 cavalry rout Pompey’s 5-8,000 cavalry. The victorious cavalry and fourth line were then free to attack the exposed flank of Pompey’s raw recruits and subsequently routed the whole army.
Another case of subtle formation shifting is Scipio Africanus deployment at the battle of Ilipa during the Iberian campaign of the Second Punic War. At this period it was more common for the most elite infantry to occupy the center and push through the middle of the enemy formation.
At Ilipa Scipio marched his men out in this battle formation, Romans in the center and his less reliable Spanish mercenaries on the wings, but did not attack and returned to camp. The opposing commander Hasdrubal matched this formation and day after day this repeated until Scipio marched out with one important change.