The Oaths of Strasbourg were a military pact made on 14 February 842 by Charles the Bald and Louis the German against their older brother Lothair I, the designated heir of Louis the Pious, the successor of Charlemagne. One year later the Treaty of Verdun would be signed, with major consequences for Western Europe's geopolitical landscape.
Louis the German swore his oath in Romance so that the soldiers of Charles the Bald could understand him. Likewise, the latter recited his in Germanic so that Louis's soldiers would understand.
The Romance section of the Oaths is of special importance to historical linguistics, as it is the oldest extant document in France that was written deliberately and consistently in a form of Romance.
Centuries after the fall of the Western Roman Empire, Charlemagne, who had conquered much of its former territory, announced its restoration. Upon his death, he passed this realm to his son Louis the Pious, who would in turn pass it to his firstborn son Lothair I. However, the latter's brothers—Charles and Louis—refused to recognize him as their suzerain. When Lothair attempted to invade their lands, they allied against him and defeated him at the Battle of Fontenoy in June 841. Charles and Louis met in February 842 near modern Strasbourg to affirm their alliance by swearing a joint oath against Lothair. The following year the civil war would end with the Treaty of Verdun, in which the three claimants partitioned the Empire amongst themselves.
The Oaths were not preserved in their original form; they were instead copied by the historian Nithard, another grandson of Charlemagne, in a work titled De Dissensionibus Filiorum Ludovici Pii "On the Quarrels of Louis the Pious's Sons". This was a firsthand account, as Nithard had campaigned alongside his cousin Charles the Bald. It was however biased, reflecting the perspective of the allies and casting Lothair as an aggressor and villain.
Louis and Charles swore their oaths not as kings—a term which is never used—but rather as lords, with their respective entourages acting as witnesses.