The Battle of Vanaheim was an open confrontation betwen the Marauders and the combined forces of the Asgardians and the Vanir and the final battle of the Marauders' War.
Following the destruction of the Bifrost Bridge at the hands of Thor in the Duel at the Bifrost Bridge, the Asgardians were unable to intervene in attacks on the other Realms, including the Marauders' onslaught on Vanaheim. The peaceful Vanir stood no chance as their planet fell into a hopeless war.
Hogun, a Vanir who was residing in Asgard and was working as a member of the Warriors Three, was told by Thor that the Marauders were attacking. Fearing for his people, he wanted to help in any way possible.
In order to bring the Bifrost back into full power, Odin sent Thor to Midgard through dark energy, where the Tesseract could be found.
Loki had also been searching for the cube, leading to the Chitauri Invasion. During which, Thor allied himself with S.H.I.E.L.D., who put him in the Avengers. The team fought against Loki and an army of Chitauri in the Battle of New York. After Loki was finally defeated, Thor was given the Tesseract and Loki as a prisoner.
On their return to Asgard, Loki was imprisoned in the Asgardian Dungeons and the Tesseract was given to Heimdall so that he could fix the bridge.
Once it was repaired, the Asgardians were finally able to travel to the worlds that were taken by force.