Kingdom of Soissons in the context of "Battle of Soissons (486)"

Gaul (Latin: Gallia) was a region of Western Europe first clearly described by the Romans, encompassing present-day France, Belgium, Luxembourg, and parts of Switzerland, the Netherlands, Germany, and Northern Italy. It covered an area of 494,000 km (191,000 sq mi). According to Julius Caesar, who took control of the region on behalf of the Roman Republic, Gaul was divided into three parts: Gallia Celtica, Belgica, and Aquitania. Archaeologically, the Gauls were bearers of the La Tène culture during the 5th to 1st centuries BC. This material culture was found throughout Gaul and as far east as modern-day southern Poland, Slovakia, and Hungary.

Warbands led by the Gaul Brennos sacked Rome in 387 BC, becoming the only time Rome was conquered by a foreign enemy in 800 years. However, Gallia Cisalpina was conquered by the Romans in 204 BC and Gallia Narbonensis in 123 BC. Gaul was invaded after 120 BC by the Cimbri and the Teutons, who were in turn defeated by the Romans by 103 BC. Julius Caesar finally subdued the largest part of Gaul in his campaigns from 58 to 51 BC. Roman control of Gaul lasted for five centuries, until the last Roman rump state, the Domain of Soissons, fell to the Franks in AD 486.

A rump state is the remnant of a once much larger state that was reduced in the wake of annexation, occupation, secession, decolonization, a successful coup d'état or revolution on part of its former territory. In the last case, a government stops short of going into exile because it controls parts of its remaining territories.

Syagrius (c. 430 – 486 or 487 or 493–4) was a Roman general and the last ruler of a Western Roman rump state in northern Gaul, now called the Kingdom of Soissons. Gregory of Tours referred to him as King of the Romans. Syagrius's defeat by King of the Franks Clovis I is considered the end of Western Roman rule outside of Italy. He inherited his position from his father, Aegidius, the last Roman magister militum per Gallias. Syagrius preserved his father's territory between the Somme and the Loire around Soissons after the collapse of central rule in the Western Empire, a domain Gregory of Tours called the "Kingdom" of Soissons. Syagrius governed this Gallo-Roman enclave from the death of his father in 464 until 486, when he was defeated in battle by Clovis I.

Historians have mistrusted the title "Rex Romanorum" that Gregory of Tours gave him, at least as early as Godefroid Kurth, who dismissed it as a gross error in 1893. The common consensus has been to follow Kurth, based on the historical truism that Romans hated kingship from the days of the expulsion of Tarquin the Proud; for example, Syagrius's article in the Prosopography of the Later Roman Empire omits this title, preferring to refer to him as a "Roman ruler (in North Gaul)". However, Steven Fanning has assembled a number of examples of rex being used in a neutral, if not favorable, context, and argues that "the phrase Romanorum rex is not peculiar to Gregory of Tours or to Frankish sources", and that Gregory's usage may indeed show "that they were, or were seen to be, claiming to be Roman emperors."

Aegidius (died 464 or 465) was the ruler of the short-lived Kingdom of Soissons from 461 to 464/465. Before his ascension he was an ardent supporter of the Western Roman emperor Majorian, who appointed him magister militum per Gallias ("Master of the Soldiers for Gaul") in 458. After the general Ricimer assassinated Majorian and replaced him with Emperor Libius Severus, Aegidius rebelled and began governing his Gallic territory as an independent kingdom. He may have pledged his allegiance to the Eastern Roman emperor Leo I.

Aegidius repeatedly threatened to invade Italy and dethrone Libius Severus, but never actually launched such an invasion; historians have suggested he was unwilling to launch an invasion due to the pressure of the Aquitanian Goths, or else because it would leave Gaul exposed. Aegidius launched several campaigns against the Visigoths and the Burgundians, recapturing Lyon from the Burgundians in 458, and routing the Visigoths at the Battle of Orleans. He died suddenly after a major victory against the Goths; ancient historians say that he was assassinated, but do not give the name of the assassin, whereas modern historians believe it is possible that he died a natural death.