Harold himself is somewhat obscure; the historian Frank Stenton considered it probable that his mother Ælfgifu was "the real ruler of England" for part or all of his reign. Kelly DeVries points that during the High Middle Ages, royal succession in Northern Europe was determined by military power. The eldest son of a king could have a superior right of inheritance but still lose the throne to a younger brother, or other junior claimant, possessing greater military support. Harold managed to win the throne against the superior claim of Harthacnut in this way. The 11th century provides other similar examples. Magnus I of Norway (reigned 1035–1047), who wasn't a warlord, had reigned for more than a decade when his uncle Harald Hardrada (reigned 1047–1066) challenged his rule. With Harald being a famous military leader, his claim would end Magnus' reign early. Baldwin VI, Count of Flanders (reigned 1067–1070) was effectively succeeded by his brother Robert I (reigned 1071–1093), rather than his own sons. Robert Curthose, Duke of Normandy (reigned 1087–1106) lost the throne of England to his younger brothers William II (reigned 1087–1100) and Henry I (reigned 1100–1135).