The Duel at the Bifrost Bridge was the confrontation between Thor and Loki upon Thor returned to Asgard from his exile on Earth. Loki intended to use the Bifrost Bridge to destroy Jotunheim and exterminate the Frost Giants, while Thor, incapable of allowing a genocide, had to battle his own brother to stop him.
Loki learned that he was the son of the Frost Giant king Laufey, who abandoned him as a child to be found and raised by Odin, the king of the Asgardians. Since Thor was the "favorite son" of Asgard and destined to be its king, Loki felt rejected by both of his fathers. Loki invited Laufey to Asgard so he could kill Odin, who was incapacitated in his "Odinsleep", but Loki killed the Jotun King, doing something neither Odin or Thor would do.
Loki activated the Bifrost to begin the destruction of Jotunheim by unleashing its full power. He explained that he only wanted to be Thor's equal in Odin's eyes, but now he desired the throne because he will have accomplished what no one else has done in destroying the Frost Giants. The two began to fight. As Loki fell onto the Rainbow Bridge, Thor placed Mjølnir on his chest, making him unable to move.
Thor realized that Loki spoke the truth and began to hammer the Bifrost with Mjølnir. Loki told Thor that if the Bifrost was destroyed, Thor would never see Jane Foster again. Thor continued to pound the bridge until it collapsed. Both fell over the side, only to be caught by Odin.