The Roman army was successful making an empire because of three reasons. The first is that they built a navy. During the first war with Carthage, between 264 and 241 BC, the Romans realized that the Carthaginians had a fleet that controlled the Mediterranean Sea. Even though the Romans were not seafaring people, they built warships. They built two kinds. They were quinqueremes and triremes. They built one hundred quinqueremes and triremes. The only problem was that quinqueremes had never been used in Italy before, so their ship builders had no idea how to build one. The other reason is that the Romans invented a device. The Romans had difficulty winning sea battles. So they made a device called a raven. It is like a bridge that they flip onto the other ship. The Romans can now board the other ship and fight hand-to-hand, which they were good at. The last reason is that that they had strong, disciplined armies.
Their armies were divided into four large regiments. Each regiment was called a legion. Each legion had between 4,000 and 5000 men. Each legion was divided into thirty companies commanded by two officers. The officers were called centurions. So these are the three reasons why the Roman armies were successful making an empire.