Why did Pope Gregory IV support the coalition against Louis I in 833?

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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.

Pope Gregory IV supported the coalition against Louis I in 833 primarily because he aligned himself with Lothar I, the co-emperor and Louis's son.1

The conflict arose from a Carolingian dynastic struggle triggered when Louis the Pious granted a portion of his kingdom to Charles the Bald, who was Lothar’s half-brother. Lothar, who opposed this endowment, led a revolt against his father. By supporting Lothar in this dispute, Gregory IV became involved in the rebellion that ultimately resulted in the deposition of Louis I in 833.1