From the ninth to the early eleventh centuries, invasions of the Magyars from the east, Muslims from the south, and Vikings from the north struck western Europe. This unrest ultimately spurred greater unity in England and Germany, but in northern France centralized authority broke down and the region split into smaller and smaller political units. By the ninth century, many knights and nobles held estates (fiefs) granted by greater lords in return for military and other service. This feudal system (from the medieval Latin feodum or feudum, fee or fief) enabled a cash-poor but land-rich lord to support a military force. But this was not the only way that land was held, knights maintained, and loyalty to a lord retained. Lands could be held unconditionally, landless knights could be sheltered in noble households, and loyalties could be maintained through kinship, friendship, or wages. Mounted armored warriors, or knights (from the Old English cniht, boy or servant), were the dominant forces of medieval armies.