Why Was The Roman Army So Successful?

They were a highly disciplined fighting force. The short blade they wielded also was very punishing as well as deadly given the fighting formations used and tactics of the Roman Army. It was a devastating force until given unruly opponents that did not conduct combat conducive to Roman training. This is what set Hannibal apart from the many who lost to this force.

From the time before Christ throughout the early imperial era the strength of the Roman army lay in its infantry. These were the legions. They were units of 5,000 men all of whom were well trained and equally well equipped.
Each legion was broken up into centuries commanded by a junior officer calla centurion. There were six centuries in a cohort and 10 cohorts in a legion. There weapons were mainly a short sword, a rectangular shield, body armour and a javelin.
The discipline of the soldiers in battle gave rise to their success, allowing the generals to perform complex manoeuvres without loosing the shape or intent of the force.
The legionaries were also in charge of building bridges, roads, forts and could conduct siege warfare as well as set-piece battles. Also accompanying the Legionaries were auxiliaries. These were non-Roman soldiers recruited from other parts of the empire. These operated in blocks of 500 or 1,000 under the command of a Roman officer. They might be specialist units like the Syrian archers fighting with their favoured weapons.
Auxiliaries were paid less than legionaries and served longer but were granted Roman citizenship on discharge.

Because they trained for up to 5 hours a day and used wood in battle but metal in training.

Because the Romans were awesome.

Because they were well disciplined and organised and they fought well in their battle which brought them and empire making them a successful roman army

They had many great leaders and a good powerful army and had many good inventions such as the ballista a piece of roman artillery