In 257, amidst the chaos of civil war, Frankish invasions, religious conflicts and Sassanid attacks, the Imperium Romanum Galliarum seceded from Rome. As in OTL, it comprised Baetica, Lusitania, Hispania Tarraconensis, Aquitania, Gallia Lugdunensis, Gallia Belgica, Germania Inferior, Germania Superior, Britannia Inferior and Britannia Superior.
But while it was reconquered in 274 OTL, in this timeline, Rome underwent a revolution in the 260s, and the revolutionaries made peace with the breakaway empire. At peace with one another, the Roman and the Celtic Empire overcame the Crisis of the Third Century by applying two very different sets of changes in the political, military and socioeconomic system.
While the revolution turned the rest of the Imperium Romanum into the Second Roman Republic, ruled by commoners in the thousands of towns and peasant co-operatives, the breakaway Imperium Romanum Galliarum introduced a mild form of feudalism, devolving control to a landowning military elite which secures the empire against external threats, but continues to exclude the vast majority from any form of political participation and soon begins to turn against each other.
This timespan thus offers the opportunity of an interesting synchronous comparison of two attempts to tackle the crisis of Roman Late Antiquity - incipient feudalism, which was the path taken in OTL, and the social, economic, political and military model of the Second Roman Republic. It also makes analyses of the mutual influences of both competing systems possible. While such a comparison alone cannot explain the Celtic Revolution of the 360s, which was also owed to the turmoil caused by the (early) arrival of the Huns, it can shed some light on some factors which have contributed to it.
After the revolution, the Second Roman Republic was a federal democracy (for more detail, see Roman constitution of 263). Except for a small minority of "peregrini" (temporary refugees, foreign merchants etc.), everyone who lived in the Roman Empire was a citizen with equal voting rights. Most decisions were taken on a relatively local level (a "civitas" usually comprised a town and a rural perimeter of 20-50 km) in general assemblies (comitium civitatis). The comprehensive public services were managed by elected local magistrates with a budget set by the comitium civitatis. In these assemblies, the representatives for the federal parliament (first: Conventum Omnium Civitatum, later: Conventum Romanum, Greek: Koinon Rhomanon) were elected. The number of representatives a civitas could send depended on the amount of tax money it paid into the federal budget. Federal (or imperial) magistrates imitated the nomenclature of the First Republic, but had nothing to do with its patrician predecessors otherwise. The federal level concerned itself with great problems concerning the entire Republic, most important among them military threats. Federal magistrates kept control of the budget, minted coins, signed international contracts, and served as supreme judges.
The Imperium Romanum Galliarum (IRG), on the other hand, remained oligarchical. Especially its founder, Caesar Postumus, attempted to reform and stabilise the oligarchic model. To this end, he merged military and political powers in the endangered border provinces (Britannia, Germania, parts of Gallia) and created loyal followers as "duces", who could, in turn, freely devolve sovereign powers like taxation and drafting to "comes". At Lugdunum, a Senate was founded, too, but here, the senators were the duces and the duoviri of the coloniae and municipia, which had to contribute in predefined manners to the war efforts, too, and in exchange were given fiscal sovereignty. The IRG eliminated much of the central administration it could no longer afford. Nominal and actual power now coalesced in the hands of an even smaller group of landed military nobility and decurional patricians.
While the new Roman structures revived civic spirit and engagement, integrated far greater groups of people into political responsibility and channeled conflicts in such a way that they did not endanger capability of action, the IRG formalised and stabilised the concentration of power in the hands of a small elite.
The Roman revolution was carried by revolting coloni and slaves, who took over latifundia and established "societates liberorum" (a sort of cooperative) in the countryside, protesters in the decaying towns and cities, and persecuted Christians, who were members of both and thus linked the two groups and provided them with an overarching ideology.
The restoration of peace and a widely supported order allowed empire-wide trade to resurge and revived economic production and the developed market economy which had been in place in the 1st and 2nd centuries CE. In the towns, "collegia" (professional guilds) formed and acted freely now; as urban counterweights against the societates liberorum, they came to be the main type of organisation of urban citizens and exerted great influence on the decisions in many civitates. The prosperity of the towns and their collegia, together with the egalitarian, but relatively hermetic societates liberorum in the countryside (when compared to the absentee landowners, who had welcomed any cheap workforce), stopped the trend towards de-urbanisation. The political links between peasantry and townspeople, who only together formed a "civitas", opened up the innovative potential of urban craftsmen and educated inventors for use in the agricultural and mining businesses undertaken in the countryside. The increased role of the collegia led to a formalisation of the job market, which went hand in hand with an institutionalisation of (vocational-professional) education. This facilitated scientific activity, raised literacy levels and contributed to linguistic (and thus also cultural) homogenisation. The first century of the Second Roman Republic is characterised by a relative social equality, which had never been achieved in Roman history before.
In the IRG, the structure of latifundia and colonate became ingrained, even though ownership was transferred in many cases from former Roman oligarchs to the new Caesars` cronies. As the fortification of towns, the building of burgi and entertainment of greater local military forces devoured resources, an internal migration occurred towards the safer Southern Gaul and Hispania. In reaction to this, the colonate was legally toughened, and coloni were bound to their estates, becoming fully indentured servants (similar to Diocletian`s reforms in OTL). The productivity of the latifundia sank in comparison to Roman societates liberorum, nevertheless, they became the new self-sufficient centres of a re-localising manorial economy. Professions exhibited no technological progress in the towns except for adaptations of Roman innovations, where these were not highly capital-intensive. In spite of bagaudae redistribution, Gallo-Roman society was highly unequal - a problem perceived by many Caesars, who attempted to remedy it through cohesive forces like the creation of a common "Celtic" cultural identity and, later, the establishment of the Celtic Church. Their effect was weak. Neither the barely superificially Latinised tribes of Aquitania, North-Western Tarraconensis and North-Western Britannia, nor Frankish and Alemannic newcomers adopted the Latin language or considered themselves primarily citizens of the IRG. As the cost of defending the North failed to sink, the safer and economically relatively stable South (Hispania Tarraconensis, Baetica, Lusitania, Gallia Narbonensis) began to develop secessionist tendencies.
The Roman revolution meant a sharp break with Roman military tradition. Legions, auxiliaries and limitanei were no longer valid distinctions, aristocratic leadership was replaced with a leadership elected by the soldiers themselves. In the Second Republic, the main division was between the self-defense forces of the civitates, which consisted mainly of conscripts serving (relatively) short terms and began to take over policing duties, too, and the well-paid and well-trained professional troops, which were mostly cavalry and navy. The latter came from all walks of life, but went through an excellent education at the Academia Collegii Militum (at first only in Alexandria), whose teachers were retired officers who had the time, leisure and sense of responsibility to devote themselves to researching options for military innovation - the first important among them during this timespan being experimental fire sypphons for ships.
The IRG was faced with less organised, but almost constant external threats from their Germanic and Celtic neighbours. Its response was to endow military leaders with economic and political power, too, and to bind them to their own piece of land, whose fortification they were responsible for. Here, too, the twin strategies of multiplying decentralised, local defensive structures and improving the cavalry, were pursued - but mostly a new nobility was entrusted with this task. Military training was less theoretical and involved closer personal ties here. Luckily for the iRG, excellent blacksmiths lived in its territories, who forged better swords for the new Gallo-Roman knights than those their Roman republican officer counterparts wielded.