The "military anarchy" of the 3rd century ended in the revolution of the 260s, in which soldiers played a vital role in overcoming old Roman political, economic, social and military leadership structures and establishing and defending the young Second Republic.
The military articles of the new constitution institutionalised the factual power of the soldiers. In the federal articles of the constitution, the decentralisation of military power was politically reigned in by creating different military forces of the autonomous civitates and of the federal level. Both are internally self-governed. The former is politically responsible to its local civitas, who also pays these local forces, while the latter responds to the federal institutions, who pay them.
The two principles of soldier democracy and federalisation were supplemented by a third one: professionalisation. To this end, the Roman tradition of excellent military training was institutionalised in war academies, run by the autonomous bodies of soldiers' self-governance (collegia militum).
The "Defensores Rei Publicae Romanae"
The "Republican Defense" were the federal armed forces. At the birth of the Second Republic, they were divided into the following sub-groups:
- Exercitus Alpinus (seat of the collegium: Cambodunum)
- Exercitus Pannonicus (seat of the collegium: Aquincum)
- Exercitus Moesicus (seat of the collegium: Naissus)
- Exercitus Asiaticus (seat of the collegium: Nicomedia)
- Exercitus Cappadocicus (seat of the collegium: Satala)
- Exercitus Syricus (seat of the collegium: Zeugma)
- Exercitus Arabicus (seat of the collegium: Bostra)
- Exercitus Aegypticus (seat of the collegium: Heraclaeopolis)
- Exercitus Africanus (seat of the collegium. Lambaesis)
- Classis Rheni (seat of the collegium: Brigantium)
- Classis Histrica (seat of the collegium: Singidunum)
- Classis Pontica (seat of the collegium: Byzantium)
- Classis Aegaea (seat of the collegium: Athenae)
- Classis Erythrae (seat of the collegium: Myos Homos)
- Classis Cyrenaica (seat of the collegium: Cyrene)
- Classis Hadriatica (seat of the collegium: Aquileia)
- Classis Tyrrena (seat of the collegium: Neapolis)
Other army and navy divisions were established later and existing ones regrouped. The seat of the Maximum Collegium Militum remained in Alexandria throughout the entire Second Republic.
The Republican Defenses were a highly skilled professional army. Its land forces consisted of well-trained cavalries and archers; they also had heavy anti-siege weaponry at their disposal. Its navy had a huge number of small and large ships for the use on rivers and on the high seas at its disposal.
Republican soldiers underwent long training in the collegium's academies: at least three years of combined strategic and/or technical studies and practical training to obtain their diploma equestris, diploma pugnatoris, diploma armorarii, diploma sagittarii or diploma nautica. During these three years, the trainee soldiers were not meant to participate in armed conflicts yet. They were compelled to work for the subsistence of their army units, though, in addition to their studies and training. Young Roman citizens could begin their military training at the age of 15. The upper age limit for commencing military training was 27.
Upon graduation, the trained soldiers were allotted to one of the divisions, where they served for twelve to eighteen years, during which they could be elected into or appointed to higher offices. After they served their terms, most soldiers went on to live in the proximity of their units, on the farms or in the workshops of the armed forces, where they would be paid their regular sold for the rest of their lives. A small number of veterans, mostly from well-educated backgrounds, undertook higher academic studies upon their retirement from active duty, usually obtaining a doctoral degree in military studies at the collegia's academies - the most prestigious one being located in Alexandria. The doctors of military studies were paid slightly higher solds and were responsible both for military research (strategies and tactics, gathering of military intelligence, research into weaponry, armour, fortification techniques, ship construction etc.) and for the training of young soldiers.
Self-governance and command structures of the Republican Defense
All graduated soldiers on active duty belonged to units of one hundred soldiers (or, in case of ships, of one ship), their "centuria". (Veterans on reserve duty, who continued to be employed by the army, were grouped into their own comitia. Veterans who left the army became voting citizens of the civitas they lived in.) They decided on practical matters of organising their camps etc. in a comitium, which usually met weekly, but could be convened at any moment in peace time by the centurion. Every three years, each comitium chooses one "elector" who meets with the other electors from his division in a "collegium militum electorale". The electoral colleges of the armed forces` divisions then elected representatives for the "maximum collegium militum" in Alexandria. The number of representatives depended on the relative size of the divisions.
The members of the Maximum Collegium Militium were exempt from their active military duty. They permanently assembled in Alexandria. They functioned as a collective generality and admiralty; they appointed the "duces", who led the army`s subdivisions and who, in turn, appointed centuriones, captains and other officers. The MCM received a global annual budget from the Federal Treasury; its exact amount being determined by the Conventum Romanum on the initiative of the Consuls (not a Censor or Aedil, as was the case of all other public services). It distributed this budget as it saw fit, determining the solds of the different ranks of soldiers and the amount of investment in weaponry, etc. The MCM had a permanent liaison officer with the Conventum Romanum and was host to a similar parliamentary emissary from the Conventum, who resided with them in Alexandria. When the Consuls declared war, they usually met with the MCM beforehand and kept the channels of communication with the MCM permanently open.
The MCM was also responsible for restructuring the divisions and relocating them.
The "Vigilia Civitatum"
In addition to the professional republican defenses, towns and cities across the Empire also maintained local armed forces, most often named "vigilium civitatis" (= city watch).
Their structures differed greatly, depending on whether a civitas was situated near the Empire`s borders and thus permanently threatened by barbarian attack, or in the safety of Italy, Baetica, Dalmatia or Greece, where military attacks were unlikely. Generally, though, the vigilia civitatum were not professional, but consisted of conscripts. Light infantry dominated here.
To illustrate the diversity of local military structures, two influential models shall be presented below - one from a civitas in peaceful Italy (Mediolanum), one from a civitas at the Danube border (Sirmium).
The model of the "Constitutio Mediolana"
In Mediolanum, each part of town nominates two recallable "custodes" for a period of one year. The custodes are paid magistrates who are responsible for maintaining the city's fortifications, for keeping the lists of potential recruits, for organising a basic training of new, young citizens when they reach the age of military service, but also for hiring and commanding the firefighters and the police in their neighbourhood. In the unlikely case of an attack on Mediolanum, the custodes gather and elect a "princeps custodum", who leads the city`s self-defense forces for the time of the military confrontation. Should the Comitium Civitatis decide that Mediolanum's vigilia participate in a military campaign of the entire Roman Republic, then the Comitium also decides into which division of the Republican Defenses the watchmen of the city are integrated and for which period of time.
The model of the "Constitutio Sirmiana"
In Sirmium, attacks by barbarians were much more likely, at least in the late 3rd and 4th centuries. As a result, the city maintains its own standing army, consisting of 99% conscripts who serve one- or two-year-terms. The Comitium Civitatis elects two "magistri militum", who are paid officers with their own budget and authority of command over their own small military hierarchy. Here, the vigilium civitatis has nothing to do with fighting fires or hunting down thieves, tasks for which separate custodes are elected. Its budget is much larger than that of Mediolanum`s vigiles, and the magistri militum employ centuriones, captains and other officers as professional soldiers with long contracts (usually twelve years). The magistri militum lead the city`s defenses in case of an attack. Should the Comitium decide that its defense forces join the Republican Defenses in a military campaign, then they retain their autonomous orginisational structures, and the magistri militum are integrated into the divisions' chain of command, leading a sub-division of an exercitus or classis.