The decline starts at once after the death of Charlemagne (814).
Louis Débonnaire (814-840) is impotent to continue the work of his father, i.e.
to maintain the unit in the Empire.
Its sons, which want to have each one a kingdom, revolt against him and reduce it to hardest humiliations.
With its death, the elder one of its sons, Lothaire, wants to be the only Master, but the two others, Charles the Bald person and Louis the Germanic one, are linked against him, beat it in Fontanet, close to Auxerre (841), and sink it to conclude the treaty from Verdun, which shares the Empire in three shares about equal, France, Germany and Italy with a long territory between France and Germany (843).
These wars, where many men perish, support the invasions of the Norman pirates.