Why was the Roman army so successful?

The Roman army invented a method of warfare that persisited for 2,000 years. Its troops were rigorously trained and exercised, and divided into small detachments under the control of officers. Roman soldiers wore effective armour, and developed tactica that allowed them to fight sucessfully against almost any enemy. In particular, the Roman soldiers were very effective at defence. They closed ranks and protected themselves with large shields, which deflected arrows and spears, until they reached closed quarters and could use their own weapons.

Paved roads were constructed to allow the Roman armies to move very quickly to any region were trouble flared up. A network of local forts and garrisons made sure that soldiers were always available.