Following the destruction of the Rainbow Bridge at the hands of Thor in the Duel at the Rainbow Bridge, the Asgardians were unable to access the Bifrost Bridge and therefore could not 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.
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 the Rainbow Bridge could be regenerated. Once it was repaired, the Asgardians were finally able to harness the Bifrost once more to travel to the worlds that were taken by force.
After fighting other forces in the Realms for about a year, the Asgardians now had to help save Vanaheim from the final threat in the realms, the Marauders. The first to arrive were the Warriors Three, Sif, and many Einherjar soldiers. When they arrived, the Marauders had already began their incursion on the Vanir.
Hogun and Fandral fought the bandits off in the forests with many soldiers, while Sif, Volstagg, and other soldiers fought the other bandits off in the Vanir's camp grounds. Soon a Kronan Marauder, one of the Marauders' strongest allies, arrived to help his comrades, causing the Hogun, Fandral, and the soldiers to flee back into the campgrounds.
Soon, Thor arrived to aid in the battle. For a while the Asgardians had the advantage, but the Kronan Marauder finally arrived at the campgrounds. All of his allies chanted his name. He tried to intimidate Thor, but was quickly destroyed with one blow, forcing the Marauders to surrender and ending the War of the Nine Realms.
The Asgardians helped the Vanir repair their homes. Hogun chose to stay with his people for a while in order to ensure their safety. On their return to Asgard, nearly all Asgardians celebrated the end of the war. The Dark Elves would soon awaken after millennia of hibernation, starting the Second Dark Elf Conflict.
In chronological order: