The Hydrogen Bomb is an Atomic Bomb that is much more powerful than the Heisenberg Device.
Upon his return from an alternate reality, Trade Minister Tagomi brought back with him a new reel of the US testing the Hydrogen Bomb in the Bikini Atoll. He showed it to Chief Inspector Kido, who then traveled to the American Reich to show it to Obergruppenfuhrer Smith, who then defied orders and traveled to Berlin to show it to Nazi High Command. The news reel convinced the Nazis that the Japanese had had the time to build more and that a war would end the world. So in effect, the Hydrogen Bomb ended World War III before it could even start.