During Thor's - Odin's biological son - coronation, the truce was seemingly broken when the Frost Giants entered Odin's Vault and tried to steal the Casket of Ancient Winters, but not before the Destroyer interfered and thwarted the theft. Thor, enraged then swore revenge on Jotunheim and gathered Loki, the Warriors Three, and Lady Sif to accompany him on the journey to Jotunheim. Thor then asked Heimdall to keep it a secret and then Heimdall transported them all to Jotunheim.
Once they arrived on Jotunheim, they journeyed through the snow eventually making their way to Laufey. Thor questioned him about how his people were able to get into Asgard. Laufey accused Odin of being a murderer and a thief and told Thor he was just a boy. He then allowed them to go, but a frost giant antagonized Thor by calling him a little princess which enraged Thor.
Thor immediately attacked the frost giant and a fierce battle ensued between the Warriors Three, Lady Sif, Loki, Thor, and the Frost Giants as the fight continued Thor clearly was enjoying the battle. At first, Thor and the others can hold their own using their skills to defeat the giants, but Thor who is getting more engrossed in the battle calls Laufey to send out more foes, the others start to struggle. Sif is almost thrown off a cliff, Volstagg is burnt by the icy touch of one of frost giants and warns the others to avoid physical contact with the giants. In the middle of the fight one of the giants manages to grab Loki's arm but, instead of burning it, caused the skin on his arm to take on the appearance of a Frost giant's before he gets out of the creature's grip and his arm returns to normal.
When Fandral is skewered by an ice spear, Sif orders a retreat which Thor doesn't notice and continues the fight while the others flee. Laufey summons a monster to chase after them which corners them on the edge of the realm since Heimdall won't open the portal because of the risk. Thor quickly arrives to kill the monster only for the group to be surrounded by thousands of giants. When all seems lost Odin appears. Thor wants his father to fight with them but Odin tried to negotiate with Laufey saying this battle was just the act of a few "Children" but Laufey says that Thor's actions just broke the truce and prepares to attack only for Odin to teleport everyone back to Asgard.
"Through your arrogance and stupidity, you've opened these peaceful realms and innocent lives to the horror and desolation of war!"
Odin - enraged that Thor had selfishly attacked Jotunheim, sparking a war that Asgard is forced into and as a result, stripped him of his powers and banished him and Mjølnir to Earth.
Loki later revealed that he had informed a guard of their journey to Jotunheim, who in turn told Odin. During the fight, Loki began to suspect about his true heritage when the touch of one of the Frost Giants had no effect on him despite the fact that it caused frostbite on the rest. Loki raged to Odin about this fact as he fell into the Odinsleep.