The Roman Army was a highly effective war machine that constantly adapted and evolved in order to defeat new rivals and conquer new territories, ensuring Rome remained the dominant superpower for many centuries.
Why were they effective – because unlike those before them the Romans took human psychology into account. The Legions were structured so as to maximize their effectiveness in battle. They understood that soldiers under the stress of battle might panic and break formation – leading to the collapse of the line. Where the Greek phalanxes often were of a great depth with men jammed close together so that in the front couldn’t use their weapons effectively while those in rear after seeing their comrades up front get killed might panic and flee – the Legions were comprised of units only a few men deep which were spaced so as to allow the trooper to use his personal weapons without hindrance – yet be close enough to support one another. The battle-hardened soldiers were often in the rear so as to “anchor” the line in place and prevent it from breaking under pressure. Light units like cavalry and skirmishers would advance – attack – and then retreat so as to try to disrupt the enemy lines. Most important is that the Romans would never attack unless the terrain was in their favor unless they had no choice. When they moved – it was done so as to minimize them being ambushed. When they stopped – they would dig in for the day and build fortifications. They preferred to entice the enemy to attack them – thereby leaving their own fortifications if any rather than the Romans attacking a fixed position. If they had no choice – they would lay siege rather than trying to storm the walls by brute force. THAT is what made the Romans so powerful. Things like discipline and weapons can help – but it still boils down to how the troops are used that wins battles.