On the Likelihood that We'll Never Know for Sure Whether Those 9th Century Danish Forays Into Saxony Were Retaliatory In Nature (Against Charlemagne and the Franks), a Preemptive Assault (Fearing a Frankish Invasion), or Just Good Old-Fashioned Norse Aggression (Though, Yes, the Hardcore Stuff Was Yet to Come; Both In England and On the Continent)

My suspicion is that it was mostly a preemptive tactic, in that if look at the fortifications being built in Southern Denmark, the fact that they made Hedeby a garrisoned city, etc., you kind of get the notion that Gudfred and company were probably expecting trouble from the Franks......and so why not take the fight to them first? I mean, that kind of makes sense, no?