 Jersey was part of Neustria with the same Gallo-Frankish population as the continental mainland.Jersey, the whole Channel Islands and the Cotentin peninsula (probably with the Avranchin) came formally under the control of the duke of Brittany during the Viking invasions, because the king of the Franks was unable to defend them, however they remained in the archbishopric of Rouen.Jersey was invaded by Vikings in the 9th century.