Complex sentence: Upon Charles' early death in 863 his realm was divided among his brothers, Louis II and Lothair II, and when Lothair II died in 869, Lotharingia was partitioned by his uncles Louis the German and Charles the Bald in the Treaty of Meerssen.
Split into simpler sentences: Upon Charles' early death in 863 his realm was divided among his brothers. After Lothair II died in 869, Lotharingia was partitioned by his uncles Louis the German and Charles the Bald in the Treaty of Meerssen.