TWO months before the atomic bomb fell on Hiroshima, President Harry Truman secretly agreed to American war plans that would have launched the largest military invasion in history.
The objective: a full-scale, million-man attack designed to bring Japan to its knees.
While historians still debate the need to drop A-bombs on Japan, there is no argument about one possibility. If the US and Japan had locked in final combat on the empire's home islands, the fighting would have reached levels of ferocity never before seen.
Japanese wartime messages and military documents make clear that the nation's leaders were ready to sacrifice millions of soldiers and civilians, including women and children, in a desperate effort to stave off total defeat. The Japanese strategy was to inflict so many casualties by attacking vulnerable targets like US troop ships that Truman would drop his demand for Japan's unconditional surrender.
''If the invasion had taken place, it would have prolonged the war for over a year, turned Japan into a wasteland, and cost hundreds of thousands of American and Japanese lives,'' says Thomas Allen, co-author with Norman Polmar of ''Code-Name Downfall'' [Simon & Schuster, 1995], a just-published history of the final days of World War II in the Pacific. ''It would have been the bloodiest and fiercest battle in history.''
''Imagine the German casualties that would have resulted from an invasion of England, and you get an idea of how fierce the defense of the Japanese homeland would have been,'' Mr. Allen elaborates.
In preparation for a final assault on Japan, Truman ordered up 1 million soldiers, sailors, and marines, many redeployed from Europe, where the war against Germany had already been won.
Phase 1 of the ''Downfall'' assault, ''Operation Olympic,'' was to be launched on Nov. 1 against Kyushu, the southernmost of the Japanese home islands. Once taken, Kyushu was to be used as a staging ground for intensive air raids on the main island of Honshu.
Phase 2 of the invasion, ''Operation Coronet,'' was scheduled to be launched on March 1 against Honshu. To soften up the beaches, US military planners - unaware of the danger of nuclear fallout - contemplated using up to nine atomic bombs modified for ''tactical'' or ''battlefield'' use, according to Allen and Mr. Polmar.
In all, thousands of allied ships and aircraft, 5 million men on both sides, and dozens of US divisions - several times the number used to take Okinawa - would have been thrown into the months-long battle that US military planners estimated would have been the costliest in history.
From his headquarters in Manila, Gen. Douglas MacArthur warned his superiors of nearly 100,000 US casualties, including dead and wounded, in the first three months of the assault on Kyushu alone. United States Army medics independently estimated 394,000 casualties during the ''Downfall'' operation, while the Army Quartermaster Corps ordered up at least 370,000 purple hearts to decorate the anticipated dead and wounded.
For every American soldier killed, several more Japanese soldiers and civilians would have been lost, military historians say.
''We knew from our experience in Okinawa and Iwo Jima that there would have been very stiff resistance and heavy casualties, but much heavier casualties on the Japanese side,'' says Patrick Cronin, a senior research professor at the National Defense University in Washington.
With Japanese military codes long since broken, US military planners were able to track Japan's preparations to repel the first phase of the invasion, preparations that anticipated with remarkable accuracy where the main allied assault would occur.
''We had the ability to intercept Japanese codes down to the military unit level, so we could literally watch the formation of more than 20 additional Japanese divisions in anticipation of the invasion,'' Allen notes.
US military planners knew Japan would rely primarily on 2 million regular troops already on the home islands, nearly 600,000 of whom were already deployed on Kyushu by August, three months before the planned US invasion.
Besides regular troops, Japan created a civilian pool of millions of men and women trained to use everything from hand grenades to bamboo spears. The civilians were galvanized by the motto that ''killing just one American soldier will do.''
''With a sword or spear, neither swing vertically nor horizontally, but always thrust tall Yankees in their belly,'' advised one manual the Japanese war ministry issued. ''With a sickle, hatchet, heavy kitchen knife, or fireman's hook, attack from behind.''
Under the master plan for the final defense of the islands, dubbed ''Ketsu-go,'' more than 10,000 aircraft were readied to repel the invaders with what Allen and Polmar describe as an ''orgy'' of day and night attacks.
Thousands more individual suicide bombers - frogmen, missile pilots, and torpedo and speedboat drivers delivering high-explosive warheads - were poised to kill thousands of American troops before they hit the beaches.
Those who made it to shore would face withering fire from machine gun nests, booby-traps, trip-wire mines, and - as a last resort - kamikaze foot soldiers with explosives strapped to their backs. According to one historian, US occupation forces that visited the planned landing sights immediately after the Japanese surrender were awe-struck by the sight of high sea walls ''backed by gun-bristling fortifications'' that were even more formidable than those erected on Okinawa.
''The weaponry prepared for ketsu-go, the operation to crush the American landings, was prima facie evidence that the casualties were certain to be much higher against Japanese defending the familiar terrain of their sacred home islands than they had been on distant possessions and conquered territory,'' writes George Feifer in ''Tennozan: The Battle of Okinawa and the Atomic Bomb'' (Ticknor and Fields, 1992).
''We shall throw everything conceivable, material and spiritual, into the battle and annihilate the enemy landing force by fierce and bold offensive attacks,'' a decree issued by the Japanese War Ministry in April announced.
As a last resort, Japanese defenders might have used biological weapons against the invading American troops.
During the war, a Japanese germ-warfare factory in Manchuria had used human subjects to test weapons inflicting the plague, cholera, typhoid, and other diseases. Plans to deliver such pestilence included unleashing hordes of infected rats or dropping ceramic bombs each containing up to 30,000 infected fleas.
Because of the likelihood of massive retaliation, Japan had so far refrained from using biological weapons. But in a last-ditch attempt to defend the home islands and inflict maximum casualties, the Japanese might well have resorted to germ warfare, even at the cost of many more Japanese lives, historians speculate.
With or without germ warfare, it would have been ''one of the most horrible campaigns in the history of modern man,'' writes retired US Army Col. James Martin Davis, in ''Dishonoring America: The Falsification of World War II History.''