The steppe peoples quickly came to dominate Central Asia, forcing the scattered city states and kingdoms to pay them tribute or face annihilation. The martial ability of the steppe peoples was limited, however, by the lack of political structure within the tribes. Confederations of various groups would sometimes form under a ruler known as a khan. When large numbers of nomads acted in unison they could be devastating, as when the Huns arrived in Western Europe. However, tradition dictated that any dominion conquered in such wars should be divided among all of the khan's sons, so these empires often declined as quickly as they formed.