Sweyn II of Denmark
Flanders became a refuge for William the Conqueror's enemies, including Edgar Ætheling in 1075 and his rebellious son Robert Curthose in 1078/79. In 1075, Robert let the Danish fleet of king Sweyn II of Denmark use Flanders harbors in their intended expedition against England. In 1080, Robert married his daughter Adela to Canute IV of Denmark on his accession to the throne. In 1085 Robert and Canute planned a massive naval attack on England. The threat was big enough for William to hire mercenaries and lay waste to some coastal districts in order to difficult the supplying of an invading army. However, the rebellion of Canute's brother Olaf delayed the expedition, and finally Canute's assassination put a definitive end to the plan.