It is indeed quite clear that the whole mythological system expounded by Snorri, and implied in all the old Scandinavian poetry, centres on the idea of Odin as the supreme god. As such he has two important sides to his nature. On the one hand he is a war-god, who assigns victory or defeat to men, and who takes the slain warriors to live with himself in Valhall; 'he is also called Val-father, because all those who fall in battle are his chosen sons: to them he gives places in Valhall and Vingolf,' says Snorri. On the other hand he is a god of wisdom and cunning, knowing all things, and a god of poetry whom the skalds regard as the author of their art. So far as the historical evidence is strong enough to prove anything regarding Odin, it indicates that a belief in both of those aspects was really a part of the old religion. We have already seen that Adam of Bremen describes Odin as the war-god among the Swedes, and Snorri also says that the Swedes thought he often appeared to them before great battles; 'to some he gave victory, and some he invited to himself, and either lot was thought good.' This association of Odin with war, and the assignation to him of all those who were slain in battle, are very prominent in the mythical sagas, which may be accepted as representing a genuine tradition in this respect, however much the details may be due to later invention. In these sagas one also finds the connection of Odin with death by hanging, which appears in some of his poetic names, and must be regarded as a real belief. The purely historical evidence is, however, very limited. Perhaps the only mention of an actual offering to Odin is that found in the account of Earl Hákon's doings after he had, under compulsion, accepted Christianity in Denmark in 975. On leaving that country, he sailed round to the east coast of Sweden, landed there, and made a great sacrifice. 'Then two ravens came flying and croaked loudly, and the earl thought it certain that Odin had accepted the sacrifice, and that he would have success in fighting.' It is very probable, however, that sacrifices to this god were more common among the Danes and Swedes than among the Norwegians, and that this may account for the lack of reference to them in the Icelandic writings.