What was the name of the agreement in 843 that split Charlemagne’s empire?

The story behind the answer

In 843, Charlemagne’s grandsons—Lothair I, Louis the German, and Charles the Bald—ended civil war by partitioning the Carolingian Empire into three realms: West Francia, East Francia, and a central strip (often called Middle Francia, including Lotharingia and Italy). The settlement also confirmed Lothair’s imperial title and created enduring political frontiers.

Its impact was immense: West and East Francia evolved into medieval France and Germany, while the middle kingdom fragmented over time. Later agreements tweaked the balance: Prüm (855) divided Lothair’s share among his sons; Meerssen (870) redivided the middle lands after Lothair II’s death; Ribemont (880) ultimately assigned Lotharingia to East Francia. The 843 pact was the pivotal first breakup of Charlemagne’s unified empire, setting the stage for Europe’s later political map.