Wikipedia
The Suessiones were a Belgic tribe of western Gallia Belgica in the 1st century BC, inhabiting the region between the Oise and the Marne, around the present-day city of Soissons. They were conquered in 57 BC by Julius Caesar.
Pliny the Elder apparently gives their name as 'Suaeuconi'.
Caesar recounts in his Gallic Wars that in 57 BC the Suessiones were ruled by Galba, and that in living memory of that time their king Diviciacus had exercised sovereignty over most of the Belgians and even parts of Britain.
Coinage minted by Belgic Gauls first appeared in Britain in the mid-2nd century BC with the coinage now categorized as the "Gallo-Belgic A" type. Coins associated with King Diviciacus of the Suessiones, issued near or between 90 and 60 BC, have been categorized as "Gallo-Belgic C." Finds of this issue of coin extend from Sussex to the Wash, with a concentration of finds near Kent. A later issue of coin, "Gallo-Belgic F" (c. 60-50 BC), has concentrated finds near Paris, throughout the lands of the Suessiones, and the southern, coastal areas of Britain. These finds lead scholars to suggest that the Suessiones had significant trade and migration into Britain during the 2nd and 1st centuries prior to Roman conquest.
Caesar describes the Belgae as going to Britain looking for booty: "The inland part of Britain is inhabited by tribes declared in their own tradition to be indigenous to the island, the maritime part by tribes that migrated at an earlier time from Belgium to seek booty by invasion. ..."
Caesar mentions that their capital was Noviodunum ("new hill-fort"), the present-day city of Soissons. Soissons was the capital city of the Merovingian Kingdom of Soissons from 511 to 613. Soissons was the birthplace of the Frankish Prince Charlemagne in the year 747, son of King Pippin the Short and Bertrada of Laon. It is today the capital of the département of the Aisne, in the northern part of Champagne.
The region is still commonly referred to as the Soissonnais and people of the region are called Soissonaires.