The structural history of the Roman military concerns the major transformations in the organization and constitution of ancient Rome's armed forces, "the most effective and long-lived military institution known to history." From its origins around 800 BC to its final dissolution in AD 476 with the demise of the Western Roman Empire, Rome's military organization underwent substant...
The military was a highly organized institution. There was a clear-cut system of rank, and a number of different divisions of the basic unit, the legion. There were about 30 legions. The legions were numbered, but the numbers tended to repeat themselves. At one point, there were 5 legions numbered III.
The organization and structure of the Imperial Roman Army, the Legion, the Praetorians, the Vigiles, the Navy, the Urban Cohort and the Auxiliaries.
Roman leadership was mixed, but over time it was often effective in securing Roman military success. The influence of Roman military and civic culture, as embodied particularly in the heavy infantry legion, gave the Roman military consistent motivation and cohesion. 
Stucture of the Roman Army The Roman army was broken down into different groups to have a clear chain of command during battle. The smallest unit was the conturbenium, which was a group of eight soldiers. These men marched together and shared a tent or a room at a fort.
A 5th Century training manual for the organization, weapons and tactics of the Roman Legions. Vegetius's "De Re Militari" was the only major work of Roman military science to survive from classical times. It was widely studied in the Middle Ages and was a key source for Medieval warfare and siege tactics.
The hierarchical structure that the roman army represents today finds its roots in quite old eras. Roman army has always stayed popular due to its acts of braveries, the excellence of its soldiers and the structure in which it is arranged that is known as Roman military hierarchy.
Rank structure of the imperial Roman army. The Roman ranking structure is such a frequently asked question that it merits a page of its own. Note that on this page only the Latin terms are used whereas much of the source material for the Roman army is actually in Greek.
The growth of the Roman Republic, and the advent and expansion of the Roman Empire were greatly dependent on Rome's military might. For centuries the Roman army was the most fearsome fighting force on the western hemisphere, eventually bringing most of Europe, the Middle East, and northern Africa under the control of Rome.