STAGE 1: Probatio - An inspection/ interview was conducted under the authority of the governor.
Criminals, slaves and non-citizens were prohibited from enlistment in the legion although non-citizens were later allowed.
In times of need, the citizenship requirement was waived. These inductees were recognized as being born ‘in castris’, accorded membership in the Pollian tribe and given a Roman name. (Recorded in accordance with the lex Iulia municipalis as nomen, praenomen, father (or former master if a freedman), tribe, and cognomen.) Non citizens granted citizenship assumed the nomen of the reigning emperor i.e., a man inducted in the reign of Tiberius might be named Tiberius-Aulus-in castris-Pollian-Longinus.
The minimum age for recruits was 17 and over (as per the legislation of Gaius Gracchus in 123 BC), although exceptions were made.
Candidates bearing letters of recommendation/introduction (from their fathers or someone with prior/current service) were more favourably received.
A medical examination was conducted, and Vegetius says that "the young soldier...ought to have a lively eye, should carry his head erect, his chest should be broad, his shoulders muscular and brawny, his fingers long, his arms strong, his waist small, his shape easy, and his legs and feet rather nervous than fleshy." Defects that did not hinder one’s ability to fight were ignored.
A minimum height of 6’ was preferred for the 1st Cohort & 5’10” for all others [modern equivalents are 5’10” and 5’8”]
Distinguishing marks (scars, etc.) were noted and recorded.
Serving soldiers could not be married and recruits had their marriages annulled.
Reading, writing, and mathematics skills (i.e., bookkeeping) were prized.
STAGE II: Eligible recruits then took a military oath or sacramentum.
An individual, generally an officer of high rank, recited the oath aloud (praeiuratio - to swear).
The recruits came forward and repeated it. Most recruits were sworn in as part of a large group at the annual renewal of the oath.
The text of the pledge to the emperor has not survived. The obligatory oath was renewed annually on January 03 under the Flavians. In general, it included promises:
To obey orders
Not to break the law
Not to desert
Not to flee the battlefield or to abandon one's place in the battle-line except to recover or fetch a weapon, save a friend or strike an enemy and
To brave death in the service of the state/emperor
The oath was repeated by one soldier and the others present all said “idem in me”.
Although it has been suggested that recruits were issued with a signaculum (a precursor of the modern-day dog tag comprised of a leather pouch worn around the neck with an inscribed lead tablet) no examples have ever been found, with the exception of those related to physical property (inclusive of slaves).
Vegetius states that inductees also received an indelible military mark (perhaps a tattoo or brand on the hand) to discourage desertion.
STAGE III: Recruits received a pay advance (viaticum) , typically three pieces of gold, to finance travel to their designated units. They were generally escorted in groups by military detachments. Recruits were assigned to specific units. The unit leaders were notified by a letter which introduced the recruit(s) and identified any distinguishing marks. The names of the recruits were then formally inscribed on a nominal role, and they were recognized as soldiers.
THE BENEFITS OF ENLISTMENT IN A ROMAN LEGION
The Roman legions offered a career in which you could:
learn a trade that you could transfer into civilian life,
receive excellent health care,
enjoy a great and varied diet,
travel to distant parts of the world and
have access to bath houses and exercise facilities.
After 25 years of service
you were granted an honorable discharge
you received a cash/land donative (3,800 denarii from Augustus to Claudius; 5,000 under Caligula)
you could marry.
Approximately half of all soldiers survived to collect retirement benefits.
Note: auxiliary units were manned by non-citizens; their veterans were issued bronze diplomas in which their citizenship was formally confirmed, and they were granted the right (conubium) to marry citizens or non-citizens.