Japanese officials, center, stand in a group facing representatives of the Allied armed forces prior to signing the surrender agreement Sept. 2, 1945, on the deck of the USS Missouri in Tokyo Bay, during a ceremony marking the end of World War II.

As the United States dropped its atomic bombs on Hiroshima and Nagasaki in August 1945, 1.6 million Soviet troops launched a surprise attack on the Japanese army occupying eastern Asia. Within days, Emperor Hirohito’s million-man army in the region had collapsed.

It was a momentous turn on the Pacific battleground of World War II, yet one that would be largely eclipsed in the history books by the atomic bombs dropped on Hiroshima and Nagasaki in the same week 65 years ago. But in recent years some historians have argued that the Soviet action served as effectively as — or possibly more than — the A-bombs in ending the war.

Now a new history by a professor at University of California, Santa Barbara seeks to reinforce that view, arguing that fear of Soviet invasion persuaded the Japanese to opt for surrender to the Americans, who they believed would treat them more generously than the Soviets.

Japan’s forces in northeast Asia first tangled with the Russians in 1939 when the Japanese army tried to invade Mongolia. Their crushing defeat at the battle of Khalkin Gol induced Tokyo to sign a neutrality pact that kept the USSR out of the Pacific war.

Tokyo turned its focus to confronting U.S., British and Dutch forces instead, which led to the Pearl Harbor attack on Dec. 7, 1941.

But following the German surrender on May 8, 1945, and having suffered a string of defeats in the Philippines, Okinawa and Iwo Jima, Japan turned to Moscow to mediate an end to the Pacific war.

However, Soviet leader Joseph Stalin had already secretly promised Washington and London that he would attack Japan within three months of Germany’s defeat. He thus ignored Tokyo’s plea, and mobilized more than a million troops along Manchuria’s border.

Operation August Storm was launched Aug. 9, 1945, as the Nagasaki bomb was dropped, and would claim the lives of 84,000 Japanese and 12,000 Soviet soldiers in two weeks of fighting. The Soviets ended up just 30 miles from Japan’s main northern island, Hokkaido.

“The Soviet entry into the war played a much greater role than the atomic bombs in inducing Japan to surrender because it dashed any hope that Japan could terminate the war through Moscow’s mediation,” said Tsuyoshi Hasegawa, whose recently published “Racing the Enemy” examines the conclusion of the Pacific war and is based on recently declassified Soviet archives as well as U.S. and Japanese documents.

“The emperor and the peace party (within the government) hastened to end the war expecting that the Americans would deal with Japan more generously than the Soviets,” Hasegawa, a Russian-speaking American scholar, said in an interview.

Despite the death toll from the atomic bombings — 140,000 in Hiroshima, 80,000 in Nagasaki the Imperial Military Command believed it could hold out against an Allied invasion if it retained control of Manchuria and Korea, which provided Japan with the resources for war, according to Hasegawa and Terry Charman, a historian of World War II at London’s Imperial War Museum.

“The Soviet attack changed all that,” Charman said. “The leadership in Tokyo realized they had no hope now, and in that sense August Storm did have a greater effect on the Japanese decision to surrender than the dropping of the A-bombs.”

In the U.S., the bombings are still widely seen as a decision of last resort against an enemy that appeared determined to fight to the death. President Harry S. Truman and U.S. military leaders believed an invasion of Japan would cost hundreds of thousands of American lives.

American historian Richard B. Frank has argued that as terrible as the atomic bombs were, they saved hundreds of thousands of American soldiers and millions of Japanese troops and civilians who would have perished if the conflict had gone on until 1946.

“In the famous words of Secretary of War Henry Stimson, (the bombs) were the ‘least abhorrent choice’ of a dreadful array of option facing American leaders,” he said in an interview. “Alternatives to the atomic bombs carried no guarantee as to when they would end the war and carried a far higher price in human death and suffering.”

Frank, who is writing a three-volume history of the Pacific war, said he continued to disagree with Hasegawa on the relative importance of the Soviet intervention and the A-bombs in forcing the surrender decision.

But he said they agreed that ultimate responsibility for what happened lay with Japan’s government and Hirohito, who had decided in June to draft almost the entire population, men and most women, to fight to the death.

“Since no provision had been made to place these people in uniform, invading Allied troops would have not been able to distinguish combatants from noncombatants, effectively turning each village in Japan into a military target,” Frank said.