Treaty of Mersen

Geopolitical divisions according to the Treaty of Meerssen:     West Francia of Charles the Bald     East Francia of Louis the German     Kingdom of Italy of Emperor Louis II

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. The eldest Louis II received Italy and the Imperial crown, the second Lothair II the Rhineland with the Low Countries, then called Lotharii regnum or Lotharingia, and the third Charles of Provence received Burgundy and Provence (territories roughly corresponding with the slightly later Kingdom of Arles).

Nevertheless Lothair's kingdom existed only fourteen years; when he died in 869 the brothers of his father, Charles the Bald and Louis the German, stepped in to divide Lotharingia among themselves. Louis II sought at least a piece of the partition, but, though crowned emperor and supported by Pope Hadrian II, was denied by his uncles. Their contract of 870 at Meerssen replaced the Treaty of Verdun, 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. Large parts of the Frisian coast however were under Viking control and therefore only divided on paper.

The borderline 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. Nevertheless it did not endure more than ten years: Upon the death of Louis the German in 876 his brother Charles the Bald camapaigned 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 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. Through the Middle Ages the western boundaries of Lotharingia marked the border between the Kingdom of France and the Holy Roman Empire until the réunion policy of Louis XIV.