The Vikings and the Children of Ash and Elm

In the year 834 Dorostad the wealthy emporium at the fork of the rye and about 100 kilometers from the dutch coast was attacked and burnt apparently by a force from denmark. The same was to happen every single summer for the next four years in the face of ineffectual frankish responses that included failed peace negotiations. There's a lot of plausible deniability built in here these rulers can use the fact that they don't control everybody with an iron fist to sort of fall bar for responsibility for this stuff.