Efficient field and military formations, formidable fighting skills, a domineering infantry, genius garrison, arms, and armaments engineers and keenly crafted Roman weapons, are some of the features of the Roman army. This elite force not only consisted of Roman citizens but also of mercenaries who fought for wages. Among all these sophisticated features of the Roman army, the highly advanced structure of the army was a big contributing factor to the success of the force. The might of the army helped the Roman empire to rule a substantial part of central Europe, some part of Asia and also a part of Northern Africa, dominating the regions till its fall. On the whole the property and success of Roman civilization was largely aided by the army's formidable campaigns. Roman Army Corps and Field Formations The army ranks which we know as of today, are an evolution of several reforms that were initiated after the blunders by Roman generals. Important lessons learned were during the wars against Carthage where Hannibal inflicted several losses on the Roman army in 216 B.C. Another incident in 9 A.D., where three Roman legions were slaughtered by the Germanic tribes in the ambush of Teutoburg forest, a loss too overwhelming. In 107 B.C., the Marian reforms gave birth to the initial and basic structure and ranks of the Roman army. At the height of its power and glory, the Roman army was divided into 3 primary corps, namely, the Roman legions, Praetorian Guard and the Roman auxiliaries. The Roman cavalry is often considered to be a separate corps, however they were integrated into the field formations of the aforementioned corps. The basic armed unit was the legion and usually consisted of 6,000 soldiers, including officers. These men were divided into cohorts, which were further divided into centuries. The cohorts and centuries were commanded by the centurions and all the senior officers in turn commanded the centurions. The primary field formation of the auxiliaries on the other hand was regimental. The auxiliaries were recruited from tribes, non-citizens, people from conquered Roman provinces. In general, they acted as mercenaries as compared to the Roman legions. Depending upon the deputation of auxiliary regiment and the need of the provinces strategic defenses, the ranks and field formations greatly differed. The auxiliaries consisted of three primary corps, namely, Cohortes (infantry), Alae (cavalry) and Cohortes equitatae (cavalry and infantry). These troops also often provided support such as logistics, patrolling, continuous watch, etc. They often acted as the paramilitary forces of Rome. There were some other corps of the auxiliaries, namely, heavily-armoured lancers, camel troops, scouts, and slingers. The Praetorian guard was an elite force, which was under the direct command of the Emperor or the Generals. The primary task of the Praetorian guards was to act as bodyguards but the probability that this guard engaged in commando and covert operations cannot be dismissed. The history of the guard is shrouded with controversy as the guard also had a political arm. Roman Army Ranks in Order The following are tables depicting the ranks of the Roman army, classified as per the corps mentioned above. It must be noted that the ranks are in descending order:

Roman Legions
Senior Officers - Starting from Senior most
Sr.No	Name of Rank/Position	Note
1.	Legatus legionis or Legate	Legion commander, holding political authority, usually a senator with military experience, governor or head of the province, multiple legions under command, commanded an entire Legion of 6,000 men
2.	Dux or Leader	General of more than one provincial military unit
3.	Tribunus laticlavius or the Broad Band Tribune	Second in command of the legion, deputy or second in command of Legate or Dux
4.	Praefectus castrorum	Third in command and also usually a war hardened veteran, formidable field commander
5.	Tribuni angusticlavii or Narrow Band Tribunes	Every legion had about 5 or so Tribuni angusticlavii, who in most of the cases were members of high ranking families, and were quite young
Centurions - Starting from Senior most
Sr.No	Name of Rank/Position	Note
1.	Primus pilus or 1st Centurion	The senior most among all centurions and led the 1st century of the first cohort.
2.	Pilus prior	The next 9 Centurions younger and inexperienced to the Primus pilus
3.	Primi ordines	The next 5 Centurions, younger by experience to the Pilus priors
4.	Other Centurions	Centurions with lesser experience with 60 such centurions being attached to one legion
Other Ranks - Starting from Senior most
Sr.No	Name of Rank/Position	Note
1.	Optio	One deputy of each centurion again about 60 per legion, appointed by receptive centurions
2.	Tesserarius or Guard commander	Second in command and one for every century (100 men) and also acted as administrative assistant
3.	Decurio	Commander of a small cavalry unit known as eques legionis and has 10 to 30 men under his command.
4.	Decanus	Commanded 8 regular soldiers/legionaries

Praetorian Guard - Descending Order
Sr No	Rank/Position	Note
1.	Praefectus	Head of all Praetorian Guards
2.	Tribuni	Deputies to the Praefectus
3.	Centuriones	Commanders of Centuries of Guards, commanded up to 100 men
4.	Evocati Augusti	Guardsmen and soldiers who chose not to retire
5.	Immunes	Soldiers with highly specialized skills, right from engineers to intelligence and assassins
6.	Milites	Regular Soldiers

It must be noted that since auxiliaries were non-regular infantry soldiers. Due to the immense complexities in the organizations, their ranks have not been included. Within the legions some special duty ceremonial posts. These included, Aquilifer, Signifer, Cornicen, Imaginifer, Immunes, Evocati, and Duplicarius. Every cohort or century had at least one such post.