At Tertry in 687 the Austrasian mayor Pepin of Herstal destroyed the Neustrian army of King Theuderic III and the mayor Berchar. Pepin took the king and the treasury into his own keeping and became mayor of the palace in Austrasia, Neustria and Burgundy together — the moment the Arnulfing family became the real government of the Franks.
The Battle of Tertry, fought on the Somme in 687, decided which of the Frankish kingdoms would govern the others and which family would govern them all. Pepin of Herstal, mayor of the palace in Austrasia, destroyed the Neustrian army commanded by the mayor Berchar in the name of King Theuderic III, and took both the king and the royal treasury into his own hands.
The dynasty was untouched: Theuderic III remained king, and Merovingians went on reigning for another sixty-four years. What changed was where power sat. From 687 the mayoralty of all three kingdoms was held by one man from one Austrasian family, and it passed to his descendants — to Charles Martel, to Pepin the Short, and finally to Charlemagne.
By the later seventh century the Frankish world was divided between Neustria in the west, Burgundy in the south-east and Austrasia in the east, each with a Merovingian king and each governed day to day by a mayor of the palace who controlled patronage, the royal estates and the summons to war. The mayoralty had become the prize, and the office was increasingly hereditary in a few great families.
Austrasia’s mayors came from the line later called the Arnulfings, descended from Bishop Arnulf of Metz and Pepin of Landen. Their first attempt on the throne itself had failed badly: in the 650s Grimoald put his own son on the Austrasian throne and was handed to the Neustrians and killed. Pepin of Herstal, who emerged as Austrasian leader about 680, therefore had a family memory of exactly how a usurpation ended, which is part of why his settlement after Tertry left the king in place.
Neustria under the mayor Ebroin had been the stronger party and had beaten the Austrasians near Laon around 680, killing Pepin’s colleague Martin. Ebroin was murdered soon afterwards, and the Neustrian mayoralty passed through Waratto and his son-in-law Berchar, who quarrelled with the Neustrian magnates. Some of them invited Pepin in.
Pepin marched west and crossed the Somme at Tertry, a few miles from Saint-Quentin, where the royal army under Theuderic III and Berchar met him. The sources are unusually thin even by the standards of the period: the Liber Historiae Francorum, written in Neustria about 727, and the continuations of the chronicle of Fredegar record the outcome and almost nothing else. There is no account of the ground, the formations or the losses.
The date is given as 687 by the Liber Historiae Francorum, and that is what most modern accounts use, though the chronology of these years is loose enough that 688 has been argued. The engagement itself seems to have been decided quickly; Berchar fled the field and Theuderic III was taken. Pepin did not pursue a war of extermination against the Neustrian nobility, and that restraint is a better guide to his aims than the fighting is.
Pepin kept Theuderic III as king and ruled in his name. He took the treasury, appointed his own man Nordebert to run Neustria under him, and within a year Berchar had been murdered — by his own mother-in-law’s faction, the Liber Historiae Francorum says. When Theuderic III died in 690 or 691 Pepin simply continued through his sons Clovis IV, Childebert III and then Dagobert III, each raised to the throne as a child.
He took the style dux et princeps Francorum, duke and prince of the Franks, a description of real power rather than a royal title. With the west secured he spent the next twenty-five years on the frontiers: Frisians, Alemanni and Bavarians. He died at Jupille in 714, and the succession crisis that followed — fought out between his widow Plectrude and his son by another union, Charles Martel — showed how completely the family’s position, not the king’s, was now the thing worth fighting over.
Tertry is often said to have turned the Merovingians into rois fainéants, do-nothing kings, overnight. That phrase comes from Carolingian propaganda, above all Einhard’s life of Charlemagne, written to justify the dynasty his patrons had replaced: Einhard’s king with the long hair riding in an ox-cart is a portrait drawn by the winners more than a century later.
Modern historians, Paul Fouracre in particular, read the evidence more carefully. The kings after 687 still issued charters, still gave judgment, and were still necessary — Pepin governed through them because Frankish legitimacy ran through Merovingian blood, and the one attempt to bypass it had killed his ancestor. The hollowing-out was real but gradual: it took the failed regency of 714, Charles Martel’s conquest of his own family’s position, and a papal ruling in 751 before Pepin the Short could put Childeric III in a monastery and take the crown.
The immediate consequence was the end of the Neustrian–Austrasian wars as a contest between equals. One family now controlled the armies, the estates and the appointments of the whole Frankish world, and could direct its resources outward instead of against itself.
The long consequence was the Carolingian empire. Pepin’s son Charles Martel used the united Frankish army to stop the Umayyad raid into Gaul at the Battle of Tours in 732 and to reconquer the south; Charles’s son took the crown in 751; his grandson was crowned emperor in Rome in 800. Every one of those steps was taken from the position Pepin of Herstal secured on the Somme in 687.