In 1941, during World War II, as part of the Axis Powers, the Empire of Japan attacked the United States at Pearl Harbor. One year later the Sierra Army Depot was constructed to defend the United States should Japan attack the west coast. In 1945, the United States military dropped two atomic bombs on the Japanese cities of Hiroshima and Nagasaki, the bombs being named "Little Boy" and "Fat Man" respectively. This was the first time nuclear weapons had been deployed in combat, and the only such event to occur before the Resource Wars.
After World War II, not much is known of post-war Japan and whether it progressed along a similar path as real-world Japan, but it would be in good standing with the United States circa 2077, with Pioneer Scouts codebreaker exam proclaiming Japanese as a patriotic language. Almost nothing is known about the country after the Great War, however.
In 2241, some descendants of the Yakuza, the pre-War Japanese mafia, inhabit the area around New Reno, perpetuating the tradition of their expertise in swordplay and throwing weapons. They have no known headquarters and travel in small bands, fighting against New Reno's mobsters and scouting the Wasteland around New Reno. They never succeed in entering into the city, however.
In 2277, Toshiro Kago, a samurai of the Oda clan born sometime before 1562 during the Sengoku period, was found aboard mothership Zeta by the Lone Wanderer after being abducted by aliens seven centuries earlier.
Japan is mentioned in Fallout 2, although Yakuza and the wakizashi appeared in Fallout 2, Fallout 3 and its add-on Mothership Zeta. The Fallout: New Vegas add-on Gun Runners' Arsenal, Katanas appear as weapons. The Battle of Iwo Jima is mentioned and depicted on a mural in the Museum of Freedom in Fallout 4. Okinawa is mentioned in the Fallout 76 intro. Throwing stars also appear in Fallout Tactics, and were to appear in the canceled Fallout Extreme.