Throughout the history of mankind, the Roman army has remained remembered as one of the best and most prestigious military institutions whose doctrines and successes in military-administrative possibilities are still carefully studied by historians, archaeologists as well as military experts of modern military powers. The base of its full efficiency and quality, the army acquires during the transformation of the Roman state from republican to imperial state model when undergoing a series of significant reforms whose main driver is the first Roman Emperor Octavian August. With August, Rome enters the imperial period of Principate when the whole country experiences changes in almost all its aspects, especially the army as one of the basic pillars of the Roman state. There is a rooted change in the very structure of the entire Roman army such as the reforms of the chain of command, the strictly defined conditions of military service, as well as the possibility of advancing which led to the creation of entire military careers similarly as we see in the modern armies today. As a result, the Roman army during the Principate becomes a permanent professional military power without significant competition for the next few centuries.