[expand]
The mythological history described war between Aesir and Vanir—the first war, setting precedent for all future conflicts. The causes were unclear in surviving sources, perhaps deliberately so—what matters was not who was right but what happened afterward.
The Vanir attacked Asgard, using seidr magic to breach its defenses. The Aesir responded with military might. Neither could decisively defeat the other. The Vanir’s magic was powerful but not infinite. The Aesir’s strength was great but not overwhelming. The war threatened to destroy both sides, benefiting only the giants who waited for divine weakness to exploit.
Recognizing stalemate, both families negotiated peace. The terms were sophisticated: they would exchange hostages to guarantee mutual non-aggression. The Vanir sent Njord, Freyr, and (possibly) Freyja to live among Aesir. The Aesir sent Hoenir and Mimir to dwell with Vanir.
But the Aesir hostages were deception—Hoenir appeared wise because Mimir whispered advice, but without Mimir he was indecisive fool. When Vanir discovered this, they beheaded Mimir, sending his head back to Odin. Rather than renewing war, Odin preserved Mimir’s head with herbs, setting it beside sacred well where it continued offering counsel. This transformed what could have been war-trigger into permanent peace resource—Mimir’s head became oracle consulted by both families, shared wisdom replacing shared enmity.
[/expand]