What internal changes allowed the Romans to gain external victories?

The Romans adopted everything useful from other peoples and successfully used it for their own purposes. For example, during the wars with Carthage, the Romans managed to create their own fleet and defeat the Carthaginians.

The reform of Mary made it possible to create an almost invincible army capable of withstanding a numerically superior enemy. Marius opened the way to the legions for the poorest strata of the population, to whom the state paid money. The army became a separate estate.

Later (in the 3rd century AD), the emperors did not promote representatives of the aristocracy, but professional military men who successfully command troops.