Everything we know about the so-called 'Kingdom of Soissons' comes from Gregory of Tours' Ten Books of Histories, written in the early 590s. He actually doesn't tell us very much, only that Syagrius succeeded his father to the his seat of power in Soissons, and that in the fifth year of King Clovis of the Salian Franks' reign Syagrius was defeated by the young ruler. That's it, we literally know nothing else about how his regime functioned and what it was like to live under his rule, let alone how contemporaries felt about the situation. We only have the unrepresentative words of Gregory of Tours, written a century later, which, even if pushed to their limits, still tell us very little about the situation in Soissons.

Naturally, this means that historians have spent a lot of time arguing against each other. Edward James, for example, has argued that the Franks were already dominant in the region and that Syagrius was a minor character who had perhaps revolted against Frankish hegemony. Penny MacGeorge instead sees this period as a time of warlords, in which many figures vied for power, in which Syagrius was only one man, who was perhaps given more of a place in the history books because he ended up being defeated by Clovis, the man who would dominate Gaul in the early sixth century. In both cases, there are no good reasons to see Syagrius as the ruler of a powerful Roman successor state.

Moreover, it would be difficult to say how Syagrius was more or less Romanised than his neighbours - many Franks had fought within imperial armies, and their neighbours the Burgundians were led by Gundobad, a 'barbarian' king who was also a Roman magister militum (the Master of Soldiers), whilst the Visigoths were even more integrated within imperial politics and would in 506 issue the Breviary of Alaric, a legal collection that has proved to be a crucial source of earlier Roman law. An often repeated fact is that Clovis the Frank had actually inherited some sort of Roman governor-esque authority for the province of Belgica Secunda. If this is true, then the war between Syagrius and Clovis was very much a battle between two men with proud Roman pretensions. Unfortunately, a forthcoming article by Graham Barrett and George Woudhuysen will probably torpedo this hypothesis by demonstrating that the source never referred to Belgica Secunda at all. But their conclusion is telling nonetheless, as the letter examined still demonstrates the survival of Roman political idioms under the Frankish regime, even if King Clovis was never the governor of a Roman province.

Of course, we should not underestimate the differences between the 'barbarians' and the Romans. For instance, we know that many 'barbarian' kingdom had separate laws for themselves and it is quite clear that new non-Roman kings were in charge rather than Roman officials. But we still need to add nuance to these pieces of evidence. As all late antique scholars now acknowledge, ethnic identity is tricky to pick apart and it would be difficult to simply say that a 'Frank', as referred to in the sources, would necessarily be an ethnic Frank descended from invaders from beyond the empire. So when we have legal sources describing different treatment of 'Franks' and 'Romans', are we seeing here definitive evidence of a cultural divide, or differences that were also shaped by wealth, social connections, and self-identification? Certainly, we know of Roman aristocrats throwing their lot with the 'barbarians' throughout this period, who served their new masters with the same degree of loyalty as their ancestors did for their emperors (that is to say, not very much in many cases). Centralised control by Italy was not popular in fifth-century Gaul, to say the least, and the rise of warlords, from a range of backgrounds, need not therefore be seen as evidence of a collapse of Romanness.

Finally, there is still the question of why Syagrius was described as a 'king of the Romans', or rex Romanorum in Latin. Some have interpreted this as a sign that Syagrius was going native, since the word rex, 'king', is often seen as taboo for a Roman ruler given their anti-monarchical history. Yet as Steven Fanning has demonstrated in 1992, this was absolutely not the case and Roman emperors had been described as 'kings' well before the fifth century. In this case, perhaps Syagrius had aspired to be an emperor? Maybe, but again, we only have phrasing from Gregory of Tours, who we cannot expect to be an expert on the messy world of fifth-century politics. Since the evidence absolutely does not suggest that Syagrius was in control of a significant portion of Gaul (the absurd map of his realm on Wikipedia is, well, absurd), I find it very unlikely that anyone would have taken 'Emperor' Syagrius seriously, even if he tried to act like an emperor.

TL; DR - the Kingdom of Soissons did not exist, at least not in the way most people imagined it.

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Sources:

E. James, The Franks (1988).

P. MacGeorge, Late Roman Warlords (2002)

G. Halsall, Barbarian Migrations and the Roman West, 376–568 (2007) - the arguments of James and MacGeorge might be a bit too complex to follow, especially without background knowledge, so it might be good to start with the straightforward narrative provided by Halsall in this book.

A. Gillett (ed.), On Barbarian Identity: Critical Approaches to Ethnicity in the Early Middle Ages (2002) - the debate between academics on ethnic identity in this period is famously bitter and complicated, if you want to dive in, this book might be your best bet.

S. Diefenbach and G. Müller (eds.), Gallien in Spätantike und Frühmittelalter Kulturgeschichte einer Region (2013) - provides some excellent surveys of the structural issues involved, particularly J. Drinkwater's 'Un-becoming Roman: The End of Provincial Civilisation in Gaul' and M. Kulikowski's 'Sundered Aristocracies, New Kingdoms, and the End of the Western Empire'.

S. Fanning, 'Emperors and empires in fifth-century Gaul', in J. Drinkwater and H. Elton (eds.), Fifth-century Gaul: a Crisis of Identity? (1992), 288-97 - for his collation of Roman emperors described as reges.