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Why did Lothar I oppose Louis I’s plans to give territory to Charles the Bald?

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Illuminated manuscript portrait of Louis the Pious as a Christian Roman emperor, holding a staff and shield, with Latin text arranged around and over the figure. — Louis I Louis the Pious as a Christian Roman Emperor, portrait from *De laudibus sanctae crucis* by Rabanus Maurus.

Lothar I opposed Louis I’s plans to grant territory to Charles the Bald because these revisions directly undermined the Ordinatio imperii of 817.12

The Ordinatio imperii was a decree established by Louis I to ensure the unity of the empire after his death, designating his eldest son, Lothar I, as the sole heir and emperor, while his younger brothers were to hold subordinate kingdoms under Lothar's suzerainty. When Louis I remarried Judith of Bavaria and had a fourth son, Charles the Bald, he revised this succession plan to carve out a kingdom for Charles.132

Because this revision granted land to Charles that had previously been allocated to Lothar, it threatened Lothar’s inheritance and his position as the primary successor. This conflict led to significant civil strife, including revolts by Lothar and his brothers against their father, as Lothar sought to defend his rights under the original 817 decree.1425