Louis the German
When Lothair II died in 869, his only son Hugh by his mistress Waldrada was declared illegitimate, so his only legal heir was his brother, Louis II. If Louis II had inherited Lotharingia, Middle Francia would have been reunited. However, as Louis II was at that time campaigning against the Emirate of Bari, Lotharingia was partitioned between his uncles Charles the Bald and Louis the German by the Treaty of Meerssen in 870. Louis the German took Upper Burgundy, territory north of the Jura mountains ("Bourgogne Transjurane"), while the rest went to Charles the Bald.