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How did the Oath of Strasbourg strengthen the alliance between Louis the German and 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.

The Oath of Strasbourg, sworn in 842, served as a critical political instrument that solidified the alliance between Louis the German and his half-brother Charles the Bald during the civil wars that followed the death of their father, Louis I.12

By formally confirming their partnership, the two brothers were able to effectively coordinate their military efforts against their eldest brother, Lothar I. This unified front proved decisive, leading to the defeat of Lothar’s forces and ultimately compelling him to sue for peace. The success of this alliance paved the way for the Treaty of Verdun in 843, which formally ended the civil wars and partitioned the Carolingian empire among the three brothers.12

Beyond its immediate military utility, the Oath of Strasbourg is historically significant as a political and linguistic document, containing early versions of both Old High German and a Romance language, reflecting the cultural divisions emerging within the empire.234