Many under Roman Imperium (government/rule), with some time after being conquered and newer generations having no connection to the their older kingdoms, would find themselves loyal to the status quo. While the legions existed, there tended to be more loyalty to the general that paid the soldiers rather than the polity itself (it is this reason why the commander-in-chief in the US is elected by the people, albeit without the risk of tyranny that direct democracy would create as seen in ancient pre-Roman Athens). Due to the amount of civil wars this contributed to, this was restructured and legions done away with, but it was around the same time that the germanic foederati states were founded inside of the Imperial borders (reminiscent of tumors TBH), the mass foreign populations flooding into the military service combined with the puppeting of those West Roman Emperors in late antiquity by "barbarian" kings (the very same that ruled some fo these foreigner reservations) and corrupt politicians, made for an disloyal military that when the time came and the strength of the Imperium dwindled to such an extent that they forwent tradition and set up their own kingdoms. To counter this with the historical timing of these things, you would either need to elect your nominal military commander (completely unhistorical) or root out the corruption. In the East Empire, before the twin imperium system was thereafter abolished, this was just done and the corruption conspiracy was destroyed and it allowed the Roman Empire to survive for nigh another thousand years. At the end of it, its the small things in this world that build up and make up the bigger picture that we all see and effects us. Anyway, about national unity. Again, the sharing of the Latin imperium and its military dominance is what first created this, but after this began to wane, there so happened to be a replacement that had been seeded back in the early days of the Empire, Christianity. For a long time, the Roman Empire's domination over church affairs across its borders and beyond made their strain of the faith the national unifier. It is only with the religious dissent of the early papacy (not for noble reasons mind you, look up the Donations of Cosntantine) that this unifier was splintered. After most of the Empire's land had been gone from dominating the mediterranean world to just holding out in Greece, modern-Turkey, and south Italy in the course of just less than 200 years, and Greek having been the dominant language of the east mediterranean since Alexander the Great until the Caliphates, and Latin having been 'hijacked' by 'uncultured barbarians', the Roman Empire in its medieval Hellenization became xenophobic to things not-Greek. Classical Latin was still treasured but later variants were scorned upon as children tainting a masterpiece painting. This would lead to Europe, or "the Emergent West" as Emperor Manuel Comnenus would call them, living in a different reality from the Roman Empire, and vice versa for them (this along with a clash of egos would lead to the Great Schism of 1054, a debate that wouldn't be settled until the Empire's destruction in 1204). So they had achieved nationalism, but with their military might, and after the battles of Manzikert and later Myriokephalon, the weight of its legacy, and their general advancement, they became haughty and prideful in the face of the 'child' nations that surrounded them, which didn't make them any friends aside from a distant Ruthenia (divided, pre-Russia). Ever since Venice, a former vassal of the Roman Empire, was hired by the Emperor Alexius Comnenus and their subsequent assisting in Crusader transport, Italians (westerners) had come to dominate Roman economics, and his grandson Manuel had been open to Westernization (even hosting jousts in the ancient Hippodrome circus), which to the Romans was appalling as they were just 'barbarians' and Rhomania was the holy pie in the sky, so the last Comneni, Andronicus Comnenus exterminated the foreign merchantile class in a genocide of "latins" to purify the country of these 'vampires' but the Italians and French had two revenges, one being a war and the next being the later Sack of Constantinople in the failed fourth crusade of 1204. Ever since their foundations in mythic times the Romans had been haughty and looked down upon their neighbors, even the Greeks (though to a lesser degree) of whom they by seeming-fate had shifted over to adopt and become over the course of a thousand years (a detail that will always fascinate me). They felt superior (some more than others), and for a long time backed it up (longer than anyone else in-fact, and that fact is what makes them important), but in the end they fell to the trap of death and demise that all things mortal shall succumb too, like all civilizations and even this very cosm we presently inhabit. Now, as for philosopher monarchs, the Roman Empire had a few of these. Vespasian was idealistic, but perhaps not a philosopher. Marcus Aurelias was indeed a philosopher. So too was Leo VI Makedon (the tortured soul). Many emperors after its complete Christianization ahd to dabble in theology seeing as to the theocratic nature of their job, but some were more enthusiastic about this than others. The thing is that the Roman Empire always seemed to be at war, at first to spread its own glory across the world, then to keep its economy afloat in order pay for all the people they absorbed, and later on after they lost most of this land they had to continue to fight to survive against near-constant invasions and civil wars, and so because of these things barracks emperors tend to be more common than taking time to do deep thinking while you gave military control to a general who very well could jsut take the army you gave him and jsut make himself emperor. Doesnt' mena amny fo them didn't see the value in such stuff though, so they promoted wise thinkers, so logn as they were not heretical or did not become corrupt and succumb to treason.