Vikings who settled in Northern coastal France and established themselves as a Dukedom. This ownership of a part of France led directly to the 100 years war.
Trounced (read as 'only just beat') the Saxon warlord Harold at Hastings in 1066 after the poor sod had a 900 mile forced march AND an earlier pitched battle at Stamford Bridge (the real bridge not the football ground).
Less well known for conquering Sicily at about the same time and being rather good at stuffing it up the Saracens during the 1st Crusade. Their heavy cavalry was said to be able to charge a hole through the walls of Jerusalem.
Ruthless colonial administrators and very good at building castles and cathedrals. Many of the aristocracy of England are descended from this bunch of opportunist squatters.