Treaty of Mersen

The Treaty of Mersen or Meerssen, concluded on 8 August 870, was a treaty of partition of the realm of Lothair II by his uncles Louis the German of East Francia and Charles the Bald of West Francia, the two surviving sons of Emperor Louis I the Pious. The empire of Louis the Pious, son of Charlemagne, 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, while:

Louis († 876), the second born son, received East Francia (which would evolve into the Kingdom of Germany)

Charles the Bald († 877), his half-brother, received West Francia (which would evolve into the Kingdom of France)

Upon the death of Lothair I in 855, his realm of Middle Francia was partitioned between his sons by the Treaty of Prüm:

Louis II († 875), the eldest son, received the imperial crown and Italy