Aesir and Vanir

Aesir and Vanir were two tribes of god-like beings in Norse mythology. Aesir resided in Asgard and Vanir in Vanaheim, which are two of the nine different worlds of the mythos. Both groups resembled human beings in their appearance, although some of them were able to shapeshift.

Essentially gods, people of Asgard and Vanaheim were worshipped by humans, who resided in Midgard. Aesir could hear their prayers via sacrifices and bestow boons and blessing in the visible world.

Aesir and Vanir war
Ancient Aesir were said to honor physical combat, where Vanir were more inclined to magic. Usually the two tribes got along fine enough even if there was some distrust between the two. Eventually Vanir goddess Freya wandered into Asgard and offered her magic for a payment. Asgardians were impressed, and gradually traded more and more for Freya's wondrous spells.

This escalated into Aesir and Vanir war, when Asgardians realized they had betrayed all of the virtues they held in high esteem for Freya's magic. With this rather questionable reasoning the Aesir declared war on Vanar.

War would continue for great lengths of time, and eventually wears down both parties. They form peace by exchanging hostages, and from that point onwards it becomes difficult to distinguish between the merged tribes. One way to think of it is that all Vanir are Aesir, but not all Aesir are Vanir.

In the aftermath of the war, both parties are also involved in creating the wisest human that ever lived named Kvasir. He is made by all the participants spitting into a vat.

Aesir and jötnar
The Aesir are counterpart to jötnar, race of chaotic and often monstrous creatures. The two groups often fight each other, but the nature of these conflicts are temporary. They are not in war, though this might be because the Aesir are afraid of jötnar, and jötnar are too disorganized.

Essentially Aesir are trying to keep things balanced, as the overall health of Yggdrasill, the world tree, is more important than looming threat of jötnar. Aesir can be said to be the side trying to maintain order in chaotic universe, and jötnar are the chaos and entropy that tears things apart.

Sometimes Aesir and jötnar even marry each other. This could be seen as an attempt to obtain balance through unification. The two groups are able to interbreed and bear children, but the offspring is either an Aesir or a jötunn, not a mixture of the two.

Norse pantheon (incomplete)
Odin Frigg Odr Freya Thor Sif Loki Baldur