THE ROMAN MILITARY
The Roman Legion, the ultimate military machine of the ancient world, was the catalyst that spread Roman conquest and civilization throughout the known world.  The core of the Roman legion consisted of heavily armored infantry.
Disciplined and well trained Legionaries fought in closed ranks against many warrior based armies where each man fought for personal glory.
With superior tactics and organization, designed specifically for technology such as the Gladius, the Legion was an unstoppable force for nearly a millennium.
It was among the first paid full time professional army in the world.  We will explore the Roman army, the impact, strategies and equipment, from the Republic through the fall of the Empire. 

ORGANIZATION OF THE ROMAN IMPERIAL LEGION
A full strength legion was officially made up of 6,000 men, but typically all legions were organized at under strength and generally consisted of approximately 5300 fighting men including officers.  It is difficult to determine whether non-combatants like field surgeons and clerks were included. 
The basic structure of the army was as follows:
Contubernium:  (tent group) consisted of 8 men
Centuria:  (century) was made up of 10 contubernium with a total of 80 men commanded by a centurion
Cohorts: (cohort) included 6 centurial or a total of 480 fighting men, not including officers.  In addition the first cohort was double strength but with only 5 centuriae instead of the normal 6.
Legio:  (Legion) consisted of 10 cohorts.
Additionally each Legion had a 120 man Alae (cavalry unit) called the Eques Legionis permanently attached to it possibly to be used as scouts and messenger.
Therefore the total fighting strength of a Legion had a grand total of 5,240 men not including the officers.
The basic designation of the 10 Cohors was the same throughout all the Legions.  They were arranged in battle so that the strongest and weakest units would be mixed throughout the formation maximizing morale and effectiveness.

Senior Officers of the Roman Legion
Legatus Legionis:  The overlall Legionary commander.  This post was generally appointed by the emperor, was a former Trivbune and held command for 3 to 4 years..  The Legatus was also the provincial governor and in provinces with multiple legions, each legion has a Legatus and the provincial governor had overall command of them.
Tribune Angusticiavii:  Each legion had 5 military tribunes of equestrian (knight) class citizens, handled many administrative tasks of the Legion, but still served in a full tactical command function during engagements.
Centurions:  Each Legion had 59 or 60 centurions, one to command each centuria of the 10 cohorts.  They were the backbone of the professional army and were the career soldiers who ran the day to day life of the soldiers as well as issuing commands in the field.
Immunes:  These were trained specialists, such as surgeons, engineers, surveyors and architects, as well as craftsmen.
Muilites Gregarius:  The basic private level foot soldier.
Tirones:  The basic new recruit.  A Tirones could take up to 6 months before becoming a full Milites.