Otto the Great forced Vassalage on Denmark in 950, and in 974 Harald Bluetooth tried revolting against Otto II and got chased off after his norwegian troops went home for the winter, ending with Otto II annexing Denmark into HRE. If Otto II lived for a longer time (at least long enough to dodge a regency council for Otto III), he might well have managed to beat back Bluetooth coming back for round two at a similar time as the Great Slav Uprising in 983