THERE are three things to bear in mind in trying to understand President Harry S. Truman's decision to drop nuclear bombs on Hiroshima and Nagasaki at the end of World War II.
The first is that, in the beginning of the nuclear age, there were three nuclear bombs, and one of the three had already been used in the test at Alamogordo. When Truman made his decision, only two were left. He wanted the earliest possible end to the Pacific war, and he wanted it without the need for an American ground invasion of the Japanese islands. Suppose bomb No. 2 had been a dud?
The second is that the Japanese had repeatedly during the course of war shown remarkable courage and tenacity in defending any position they held. The Americans took bitterly heavy casualties in every one of the island-hopping battles on the road to Japan. Guadalcanal, Tarawa, Iwo Jima, and Okinawa gave America's war leaders every reason to expect that a land invasion of the Japanese islands would be difficult, hard fought, and very costly in human life. Casualty estimates ran high. Adm. William D. Leahy, the president's chief military adviser, used ''half a million'' in discussing with me what the invasion might mean in United States casualties.
The third is that, in top government quarters in Washington, most people were already beginning to worry about how much the Soviets expected to gain from their extraordinary feat of driving the German Army back from the gates of Moscow and Stalingrad and along to Berlin. The Soviets were the military masters of both Eastern and Central Europe, and Joseph Stalin was showing ambitious expansionist inclinations.
In London at that time, Winston Churchill was urging an early ''Big Three'' conference to take place while most of the American Army was still in Europe as a counterpoise to the weight of the Soviet Army. A large part of that American Army was listed for transfer to the Pacific for the invasion of Japan, which had been scheduled for Nov. 1. Churchill did not want to meet Stalin when the bulk of the Soviet Army would still be in Europe and most of the Americans gone.
Truman disagreed with Churchill - one of their few differences. But his reason was the same. He wanted to wait until he knew that the bomb was about to be tested. He hoped that a successful test would add to his bargaining weight more than the departure of a few American units from Europe would take away.
The first successful bomb was tested at Alamogordo on July 16. Truman had his first meeting with Joseph Stalin at Potsdam on July 17. Truman had what he wanted - the knowledge that the bomb worked. His vigor and willingness to talk back to Stalin surprised and delighted Churchill, who had been concerned that Truman would be less willing to stand firm than Franklin Delano Roosevelt would have been.
A corollary to the above is that Truman and many of his advisers thought it would be desirable to have the atom bomb tested successfully before the Soviets entered the Pacific war. Stalin had promised Roosevelt at Yalta that they would do that. By the spring of 1945, with the Germans defeated and Japanese defeat clearly visible and inevitable, with or without Soviet help, Truman and his friends began to regret the price his predecessor had been willing to pay to get the Russians into the war.
Manchuria was only part of the traditionally Chinese territory that Stalin expected to get out of entering that war.
Nowhere else was the news of the dropping of the bomb greeted with more relief than among American infantrymen in the Pacific preparing to storm beaches of Japan. To them, it meant rescue from an ordeal that they would prefer to avoid. Many would not have survived.
But was the dropping of the bomb necessary to bring about a Japanese surrender before the invasion planned for Nov. 1?
The best authority on this subject has to be Truman's chief military adviser, Admiral Leahy. The admiral, who was also chairman of the US Joint Chiefs of Staff and the Combined US and British Chiefs of Staff, wrote in his book, ''I Was There'' (1950), the following:
''It is my opinion that the use of this barbarous weapon at Hiroshima and Nagasaki was of no material assistance in our war against Japan. The Japanese were already defeated and ready to surrender because of the effective sea blockade and the successful bombing with conventional weapons.''
Other military experts have agreed with the admiral that Japan was near surrender. Its Navy had largely been sunk. Its best pilots were killed in kamikaze attacks. Its Air Force was unable to prevent daily Allied attacks on Japan's cities and industrial centers. Its Army was repeatedly defeated. Besides, the sea blockade had shut off overseas supplies of food, oil, and industrial raw materials vital to the civilian and military economy. Japan was defeated and helpless. Some think that the use of the bomb may have shortened the war by perhaps one or two weeks.
Truman, however, himself a veteran from the previous war, was thinking about the half million or so US soldiers preparing for the invasion. He knew that the bomb would lift a load of anxiety from their backs and those of their families. And he longed for the increased bargaining power against the Soviets that he knew the bomb would give him.
There is no evidence that Truman hesitated for long, or that he was troubled or anguished over the decision. To him, the bomb represented a quick way to save a lot of American lives, and incidentally, gain new leverage in future negotiations with the Soviets.
Nor did he have much time for others who proposed dropping one bomb in such a way as to show the Japanese what we had, but without killing many civilians. One idea was to drop it in the middle of Tokyo Bay. To Truman, dropping the bomb where it would be a decisive stimulus to surrender was important.
Leahy was a great admirer of Truman and speaks with respect of him throughout his book. But in his book, he said of the bomb the following:
''These new and terrible instruments of uncivilized warfare represent a modern type of barbarism not worthy of Christian man. One of the professors associated with the Manhattan project told me that he hoped the bomb wouldn't work. I wish that he had been right.''
* Other articles ran Jan. 30, Feb. 13, March 6, April 10, May 5, June 12, and July 17. Joseph C. Harsch covered World War II for the Monitor from Washington, D.C., the Pacific, and Europe.