The Treaty of Meerssen or Mersen was a partition treaty of the Carolingian Empire concluded on 8 August 870 by the two surviving sons of Emperor Louis the Pious, King Charles the Bald of West Francia and Louis the German of East Francia, at Meerssen north of Maastricht, in the present-day Netherlands.
The empire of Louis the Pious had originally split in three parts by the 843 Treaty of Verdun, whereas his eldest son Lothair I received the Imperial crown and the personal realm of Middle Francia, including Italy, the Low Countries and the Rhineland with the Aachen residence.
In 855 Lothair I fell seriously ill and retired to the abbey of Prüm. As he aimed to defend his kingdom from the claims of his brothers, he bequested it to his three sons.
Nevertheless Lothair's kingdom of Lotharingia or Middle Francia existed only fourteen years; when he died in 869 the brothers of his father — the half-brother Charles the Bald and second born son Louis the German stepped in to divide Lotharingia among themselves, vice letting the region go to their nephews and Lothair's other heirs. Nephew Emperor Louis II and King of Italy sought at least a piece of the partition, but, though crowned co-emperor and supported by Pope Hadrian II, was denied additional lands by his uncles.
Their contract of 870 at Meerssen replaced the 843 Treaty of Verdun which had split the realm into three parts, by dividing the northern part of Middle Francia stretching from the Jura mountains in modern Switzerland to the North Sea - Lotharingia - between West and East Francia, in effect recombining sundered middle Franconia territories into two larger east and west divisions. At this time, Large parts of the Frisian coast however were under Viking control and therefore only divided on paper.
The borderline would set the shape of East Central Europe until the eighteenth century. It ran roughly along the Meuse and Ourthe rivers, allocating Brabant and Hainaut to Charles the Bald, the Rhineland and the former Duchy of Alsace to Louis the German. The arrangement did not endure more than ten years: Upon the death of Louis the German in 876 his half brother Charles the Bald campaigned against eastern Lotharingia, but was rejected by Louis' son Louis the Younger in a battle at Andernach.
In turn, after Charles the Bald had died and his successors struggled to consolidate their rule over West Francia, Louis the Younger campaigned in western Lotharingia in 879. Charles' grandsons were forced to cede the whole Lotharii regnum to him, sealed by the 880 Treaty of Ribemont, according to which Lotharingia finally became part of East Francia.
This establishment lasted throughout the balance of Middle Ages and well into the Early Modern European era, the western boundaries of Lotharingia marked the border between the Medieval Kingdom of France and the Holy Roman Empire until the 1660s–1670s réunion policy of The Sun King, Louis XIV of France (b.1638, r.1643—d.1715).
Chisholm, Hugh, ed (1911). "Mersen, Treaty of". Encyclopædia Britannica (11th ed.). Cambridge University Press.