The Greater Nazi Reich became the first country to develop an atomic bomb in 1945, as the Reich and the Japanese Empire conducted concurrent invasions of the United States on both coasts. The Nazis dropped a Heisenberg Device on Washington D.C. on December 11, 1945, destroying the city, the United States government, and much of its military leadership. However, victory was not declared until September 18, 1947, when the last of the American Resistance capitulated.
By 1962, the Greater Nazi Reich remains the only country with atomic weapons. The Japanese Empire lagged behind the Germans technologically and their own atomic weapons program was anemic. However, through the hard work of Trade Minister Tagomi and Rudolph Wegener, the Japanese obtained the schematics to build their own atomic bombs and began producing them in secret in San Francisco.
At the same time, tensions between the Japanese and Germans had intensified due to Martin Heusmann's machinations and World War III was on the verge of erupting; as only the Germans had functioning atomic bombs, the Japanese would have been defeated within days, if not hours.
Conflict was only avoided because the Nazis came to believe the Japanese possessed their own atomic arsenal plus a stockpile of more powerful Hydrogen Bombs. In 1963, the Japanese successfully detonated their first atomic bomb at Monument Valley, Utah. While this meant that the Japanese had finally achieved technological parity with the Germans, it was believed likely that the Germans would analyze data from the weapon test and realize that the Japanese lacked Hydrogen Bombs. This would trigger an arms race which the Japanese could not financially sustain for long due in part to an undeclared oil embargo that the Reich was imposing.