On August 8, 1945, Russia's foreign secretary, V.M. Molotov, summoned the Japanese ambassador in Moscow to an urgent meeting. Dispensing with the usual diplomatic niceties, he handed the Japanese envoy a declaration of war, effective the next day.
The ambassador responded with a sigh of resignation. ''The Pacific war will not be of long duration,'' he predicted.
Five thousand miles away, Gen. Douglas MacArthur, commander of United States forces in the Pacific, drew the same conclusion. Hearing of Russia's belated entry into the Pacific war he told his subordinates in Manila that Japan was now caught in a ''great pincer movement.''
''In Europe, Russia was on the eastern front, the Allies on the west,'' he noted, referring to the vise that had squeezed Germany into submission three months before. ''Now the Allies are on the east and Russia on the west, but the result will be the same.
At 1 a.m. local time on Aug. 9, 1.5 million Red Army troops, thousands of tanks, and hundreds of Soviet aircraft swarmed across the Manchurian border and engulfed Japan's vaunted Kwantung Army, for nearly 15 years the feared occupiers of the puppet state of Manchukuo. Weeks later, the Red Army moved into Japanese-occupied Korea.
The opening of a second front against Japan, long-awaited by the Western Allies, occurred three days after the first atomic bomb was dropped on Hiroshima and just hours before the second atomic bomb was dropped on Nagasaki. It ended Japan's last hopes of securing a separate peace with the Soviet Union or of winning Moscow's support for more lenient peace terms from the United States, thus sealing the fate of the Japanese empire.
''The invasion, combined with the atomic bombs, shocked the Japanese into surrender,'' notes American University diplomatic historian Robert Beisner.
The Russian invasion also had longer-term implications. Soviet military intervention in the Far East sowed seeds of future discord, contributing to eventual conflict in the Korean peninsula and to the future takeover of China by a communist regime.
Soviet leader Joseph Stalin had hinted as early as Dec. 8, 1941 - the day after Japan attacked Pearl Harbor - that the Soviet Union might enter the war against Japan.
Those hints were translated into a formal pledge by Stalin - exacted by President Franklin Roosevelt and British Prime Minister Winston Churchill at the Yalta conference in February 1945 - to strike at Japanese positions on the Asian mainland three months after the defeat of Germany.
US military planners believed that if Japanese troops in Manchuria were tied down by the Red Army, they could not be redeployed to defend the Japanese main islands against a planned US assault.
Such an assault, until the atomic bomb was successfully tested in July, was assumed to be necessary to force Japan to surrender.
''We must not invade Japan proper unless the Russian army is previously committed to action in Manchuria,'' General MacArthur told a War Department official in February 1943.
For Stalin, the opening of a second front offered the chance to make huge strategic gains on the cheap. For the price of what he estimated would be no more than three months of fighting in Asia, the Soviet leader was convinced that he could erase what he described as ''a black spot on our country'' - Russia's humiliating defeat in the 1904-5 Russo-Japanese war.
The war had stripped Russia of its dominant influence in Korea, leases in Port Arthur and the Liaotung Peninsula, railroads in Manchuria, and the southern half of Sakhalin Island, an island north of Japan. The deal at Yalta more than restored those losses and helped expunge the dark memory of defeat. ''Our people believed and hoped that a day would come when Japan would be smashed and that blot effaced,'' the Soviet leader exalted after the start of the Manchuria invasion. ''Forty years we have, the people of the old generation, waited for this day.''
In a desperate effort to keep the Soviet Union out of the Pacific war, Japanese diplomats had raced to Moscow after Germany's surrender in May 1945. They promised to renounce the gains of the Portsmouth peace treaty that ended the Russo-Japanese war, in effect offering Stalin the same territorial concessions gained at Yalta.
To the Soviet ambassador in Tokyo, Japan proposed an actual alliance that would provide the Soviets with oil, tin, and rubber from the Japanese-conquered ''southern region,'' including Malaysia.
But military victories promised far more lucrative gains for Stalin than Japanese diplomacy. According to newly released Soviet documents cited in the journal Military History Quarterly (MHQ), Stalin was finalizing plans by the spring of 1945 for an ambitious four-pronged assault that would have thrust Soviet troops onto the very mainland of Japan.
Three Soviet military objectives - Manchuria, the Kurile Islands, and southern Sakhalin - were envisioned in the Yalta agreement. But Stalin also planned to launch an invasion of Hokkaido, the northernmost of the Japanese home islands, two months before a planned American invasion of the southernmost island of Kyushu was scheduled to begin. The invasion would have violated an understanding struck at the Potsdam conference in July 1945, where the Big Three leaders (Roosevelt, Churchill, and Stalin) agreed to a demarcation line separating US and Soviet interests north of Hokkaido.
Even so, Stalin pressed on with plans to invade Hokkaido for a full week after the Japanese emperor's Aug. 15th surrender announcement, calling a halt only on Aug. 22, just one day before the operation was to begin.
If the invasion had taken place, the Soviets would almost certainly have demanded a role in the post-war occupation of Japan, much as the US, the Soviet Union, France, and Britain shared the occupation of Germany.
''Soviet control of all or part of Hokkaido would have created a situation in the Far East analogous to that which came to exist in postwar central Europe,'' David Glantz, director of the US Army's Foreign Military Studies Office, writes in the spring issue of MHQ.
''A two- (or even three-) power - the United States, Soviet Union, and Great Britain - division of Japan would have forestalled postwar Japanese reconstruction, prolonged the military occupation of the country, compounded future difficulties in Korea, and placed Japan at the focal point of a more intense cold war in northeastern Asia for decades to come,'' Mr. Glantz concludes.
As it was, Stalin backed off at the last minute following a sharp diplomatic demarche from President Harry Truman, and no Soviet troops advanced to Japan proper.
The US, meanwhile, spared a costly invasion of the main islands by Japan's surrender, occupied Japan for seven years, laying the groundwork for the defeated nation's economic and political renaissance.