Documents emerging from once-closed Soviet archives are forcing historians to rewrite the history of the last days of World War II and reassess the impact of the Hiroshima bomb on Japan's surrender.

Tsuyoshi Hasegawa, a professor of history at the University of California-Santa Barbara, said the evidence shows that it wasn't so much the bombing of Hiroshima and Nagasaki that forced the Japanese to capitulate in August 1945, but the Soviet invasion of Manchuria and impending threat of Soviet occupation of the Japanese mainland.

"I think the Soviet presence was crucial," said Hasegawa, author of "Racing the Enemy: Stalin, Truman and the Surrender of Japan."

Hasegawa, whose specialty is Russian history, said histories for the last half-century have treated the Soviet entry into the war against Japan on Aug. 8 as a sideshow. U.S. textbooks today emphasize the use of the atomic bomb on Hiroshima on Aug. 6 and the bombing of Nagasaki on Aug. 9 as the decisive action forcing the Japanese to surrender by Aug. 14.