The article is divided into two parts. The first one presents the course, the causes, and the direct consequences of the civil war of 840–843, which divided Louis the Pious’s sons: Lothair I, Louis “the German”, and Charles the Bald. Special attention is focused on the most important event of the war – the Battle of Fontenoy. In the second part of the article, the author discusses the structures within which the events took place, including the rules of succession. The author presents the disruption of two tendencies: the principle of the equal division of the country between all the sons of the deceased king, and the idea of the unity of the empire. Aspects of this issue include the successive Christianisation of state life, so that the state and the Church were almost entirely overlapping, which was the main cause of the increased significance of the idea of the unity of the empire. Another reason for this was the development of an imperial aristocracy – aristocratic families possessing manors, important positions, and interests in various parts of the Carolingian Empire. However, the idea of unity was lost as a direct consequence of the Battle of Fontenoy.