Why were the Roman armies so unbeatable?

The Romans were able to expand their territory because they had strong army which was organised into legions.

The legion had several advantages over the plalanx.It was smaller and could move faster.Soldiers in a Phalanx were used to fighting as a group and attacking only from one direction .Each legionary depended on his own fighting ability .The groups within s legion could split off from the main body and attack from sides and the rear as well as the front .

Legionaries were well trained .They spent hours practing with their double -edged iron sWords .They went on long marches every day.Befor going to sleep, they had to build comete fortified camps,even when the legion would stay in the area only one night .They built roads out of lava blocks so troops and supplies could move forward more rapidly.

-answered by Riya

The Roman army was professional and better equipped than many other armies. The military career lasted 20 years and soldiers trained and were drilled regularly. The soldiers were disciplined, well versed with warfare and were less likely buckle under pressure. Rome also had many good military commanders who later during the empire also became professionals. Roman military tactics were good. Grit and determination were also Roman military characteristics. They would go to any length, as the building of the first military road (the Appian way, which was the first stone-paved road in history) in 312 BC to speed up the movement of troops and supplies to the front of the second Samnite War and the building of a bridge across the very wide river Danube to reach the Dacians (In Romania and Moldova) who were raiding the empire from across that river demonstrate.

Additionally, Rome's military success was based on its flexibility. The same basic structure could adapt to local conditions and defeat very different opponents. Over time the Romans became skilled at raids and ambushes as any irregular tribesmen, yet preserved their superiority in massed battles.

Romans always fought aggressively and their ability to accept heavy losses and regain any lost strength made them difficult to beat.