In the years that followed the explosion of atomic bombs over Hiroshima and Nagasaki in 1945, conventional military thinking was turned on its head.

Often unbeknownst to its citizens, the U.S. government in the late 1940s and 1950s repeatedly considered using nuclear weapons. The three American military services competed fiercely to come up with possible ways to exploit the newly discovered power of the atom.

The postwar years also saw the beginning of arms control discussions (they were unsuccessful) and of what was subsequently called "nuclear diplomacy," the threat to use atomic weapons as a means of ending a U.S.-Soviet crisis.

Perhaps the greatest difference, at least in attitudes, between that era and the present was the then common assumption that use of nuclear weapons again was virtually inevitable.

The enemy against which they would be used, of course, was the Soviet Union. Immediately after Hiroshima, the Soviets embarked on a scientific crusade to build a bomb of their own.

At the United Nations, the United States made an effort to persuade the Soviets to get along without their own bomb, but that "Baruch Plan" got nowhere. Joseph Stalin was apparently bent on matching the Americans in this new category of weaponry, which seemed to promise so much power -- diplomatic as well as military -- to the countries that possessed it.

McGeorge Bundy, national security affairs adviser under President John F. Kennedy, said in a recent interview that his research in writing a history of the atomic age showed that Stalin was "absolutely determined" to get the bomb because he believed that "the Americans have upset the equilibrium" and that he needed an "equalizer."

In the United States, military men and many scientists who had worked on the Manhattan Project fully expected the next war to be nuclear, and set to work preparing for it.

Hans Bethe, for example, said in a 1979 interview that he and other scientists who worked on the bomb in 1945 at Los Alamos, N.M., thought atomic weapons would be used again within 10 years. Like some others, Bethe disliked the idea of the bomb, but he returned from private life in 1950 to help develop the hydrogen bomb, because he was fearful that Soviet development of such a weapon would put the United States at a disadvantage.

The first atomic bombs were based on the principle of fission, creating explosive energy by splitting atoms apart. In the much more powerful hydrogen bomb, the explosion is created by fusion, in which atoms are forced together, triggered by a small fission explosion. Forecast Eerily on Target

It would be wrong to suggest that the early years of the nuclear era were dominated by ignorance. In fact, many officials and military men did understand the implications of the new weapons.

Even before the first bomb was dropped on Hiroshima, some -- Secretary of War Henry L. Stimson, for example -- forecast many of the diplomatic problems with the Soviet Union that nuclear weapons would create, and that we are living with today.

One striking example was a paper delivered in January 1946, five months after Japan surrendered, by the Army chief of staff, Gen. Dwight D. Eisenhower, to the joint chiefs.

Buried until recently in the National Archives, the formerly "top secret" document outlined the Army's views on the "effect of atomic weapons on national security." It eerily predicted almost all that has occurred in the succeeding 39 years.

Eisenhower's Army staff reasoned:

If within five years there were no international agreement to prevent other nations from getting the bomb, including unlimited on-site inspections, "the world would head directly into an atomic weapons armaments race with the assurance of supremacy . . . for the nation winning that race."

The United States, which at that time had a nuclear monopoly, faced "true peril" in "a world of unrestricted atomic bombs."

"With atomic weapons, a nation must be ready to strike the first blow if needed. The first blow or series of first blows may be the last." (In today's parlance, that marked the birth of the "first strike" doctrine.)

An "atomic war will very likely be a war of surprise and surely one of national survival," but not "a prolonged one."

"Two disciplined nations each using the bomb can destroy each other's entire national life, yet neither could invade the other with large armed forces in the face of atomic bombs used on the convoys, beachheads or airheads. It makes war unendurable. Its very existence should make war unthinkable."

"Defense against the atomic bomb will always be inadequate . . . .T he only defense which we can yet foresee is to stop the carrying vehicle."

"If we were ruthlessly realistic" we would not permit any foreign power, other than an ally, "to make or possess atomic weapons."

"If such a country started to make atomic weapons, we would destroy its capacity to make them before it had progressed far enough to threaten us." (But that was a course that, Eisenhower's staff acknowledged, the U.S. government would not pursue.)

The paper concluded, "If there are to be atomic weapons in the world, we must have the best, the biggest and the most . . . ."

"All possible methods of delivery of atomic weapons, including aircraft, guided missiles, rockets and submarines should be studied and developed." Sending Nuclear Signals

The first excursions into nuclear diplomacy were conducted by Harry S Truman.

In 1948, the Soviet Union closed down land access to Berlin and Truman instituted an airlift. Faced with the prospect that Allied planes could be harassed, Truman briefly considered the use of nuclear weapons -- although the United States then had fewer than five operational bombs, according to official records.

On July 15, 1948, Truman's National Security Council (NSC) decided to send 60 B29s to Great Britain. The decision to send "atomic bombers" was given wide publicity, creating the impression that Truman was preparing to use nuclear weapons.

However, no nuclear bombs were sent, and it has since been learned that the planes were not equipped to carry them.

A year later, however, after the crisis eased, nuclear-capable bombers were sent to Britain. And in 1950, at the request of the Pentagon, Truman agreed to send the first non-nuclear components for the bombs to Britain and later to bases in the Pacific, so they would be ready for use on short notice.

The superpower situation changed sharply in 1949. On Sept. 3, a U.S. RB29 airplane, flying from Japan to Alaska on a regular intelligence flight, picked up radioactive debris in the air off the Soviet Kamchatka Peninsula. When matched with later samples, it confirmed that the Soviets had exploded an atomic device.

One immediate consequence was Truman's decision to proceed with the hydrogen bomb.

After the Korean War began on June 25, 1950, Truman had one more fling with nuclear diplomacy.

According to declassified NSC papers, the president agreed that nuclear weapons would be used only if the total defeat of the United States and other U.N. forces were imminent. And although the Chinese almost drove the Americans into the sea, Truman never turned to nuclear bombs.

One reason, according to Paul H. Nitze, who was on the State Department's policy planning staff then, was the paucity of available bombs. If nuclear weapons had been used in Korea, Nitze said recently, the United States would have been left with too few nuclear bombs to deter the Soviet Union in Europe.

In the fall of 1952, however, Truman approved a Central Intelligence Agency "disinformation" program to "spread the rumor that because of the presidential campaign candidate Eisenhower promised to go to Korea to settle the war the Truman administration was going to be forced to use atomic bombs not just in Korea but in China," according to a footnote to notes from a Sept. 3, 1952, NSC meeting.

As a newly inaugurated president in 1953, Eisenhower was frustrated by the lack of progress in the Korean peace negotiations. He pressed his military advisers to come up with ways to use nuclear weapons if the talks ended and fighting resumed against a large Chinese-North Korean force.

During a Feb. 3, 1953, NSC meeting, Eisenhower said, "We should consider using tactical atomic eapons on the Kaesong area, which provided a good target for this type of weapon," according to published notes from the meeting.

When Gen. Hoyt Vandenberg, Air Force chief of staff, suggested using nuclear bombs against air bases in China, then-Secretary of State John Foster Dulles and his brother, CIA Director Allen W. Dulles, objected. They feared that this could bring the Soviets into the war and that U.S. forces would make a better target for a Soviet nuclear attack.

A month later, Gen. Lawton Collins, Army chief of staff, reported that he was "very skeptical about the value of using atomic weapons tactically in Korea. The communists are dug into positions in depth over a front of 150 miles," he said. He pointed out that a nuclear test "last week proved that men could be very close to the explosion and not be hurt if they are well dug in . . . ."

After a presentation by the joint chiefs in May, which reported "no good strategic targets within Korea," notes from a May 13, 1953, NSC session said, "the military were most anxious to make use of atomic weapons . . . outside Korea."

In a recent interview, former president Richard M. Nixon, who attended those sessions as vice president, said Eisenhower was sending private messages to the North Koreans through John Foster Dulles, threatening use of nuclear weapons. That type of nuclear diplomacy, Nixon contended, brought a satisfactory end to the Korean War.

Bundy said he believed that Eisenhower had led his military commanders to think they would get approval to use nuclear weapons. "He didn't mind conveying the message that it could happen," Bundy said, "but he never came anywhere near facing the judgment in Korea, or indeed anywhere else, that this was something he was prepared to order." A 'More Cavalier' Attitude

The Eisenhower years saw the biggest growth ever in the nation's stockpile of nuclear weapons, from roughly 1,000 warheads when Eisenhower arrived at the White House to 18,000 when he left.

It was a time when the three services fought each other for more of the annual production of nuclear materials and encouraged scientists to develop weapons suitable for their respective use.

It was also a period when the United States started putting nuclear-capable weapons and their warheads around the world -- particularly in countries near the Soviet Union -- and undertook regular exercises of nuclear-capable forces, such as airborne bomber alerts, that would seem frightening and provocative today.

Dr. Harold Agnew, a physicist who worked on the Manhattan Project and later headed the Los Alamos National Laboratory, said recently that nuclear scientists and military planners "were much more cavalier about the use of weapons then. Fortunately, they were never used."

When Eisenhower became president, he believed that atomic bombs were a major advance over earlier weapons and had to be integrated into the military forces. He also felt that their deployment could save money by providing defense without the need for the United States to maintain large standing forces.

Eisenhower translated his belief into a national security directive that declared that, for budgetary and contingency planning, the military services were to consider nuclear weapons to be like conventional weapons. This led to the purchase of so-called dual-capable systems, such as artillery pieces that could fire conventional and nuclear shells.

Thus, nuclear versions of conventional weapons became available for use in a crisis, albeit with a requirement of presidential approval in each case. Some weapons acquired under that authority -- such as nuclear warheads for antiaircraft weapons -- were deployed during Eisenhower's administration with "predelegated authority."

That meant theater commanders could authorize their use without specific presidential approval.

The Korean War sent the Army rushing to develop tactical nuclear weapons -- small devices that troops could use on the battlefield. When the Army engineers asked for a weapon, for example, Los Alamos scientists took a Hiroshima-type bomb and "put it in a can," according to Agnew, thus producing the first atomic demolition mine. The device, which, on paper, could have closed off the three possible invasion routes from eastern to central Europe, has never been accepted by the Western Europeans for use on their territory.

Nonetheless, hundreds were built, and the Army today is attempting to develop a lightweight model that could be carried by an individual soldier.

Artillerymen pushed development of a nuclear shell and the so-called atomic cannon. When sent to Europe, they were so unwieldy that they regularly got stuck in the narrow streets of Western European towns. Lacking treads, however, they could not be used off main roadways.

To prove that they could operate on a "nuclear battlefield," the Army successfully pushed the Atomic Energy Commission (AEC) to allow it to hold exercises at the Nevada nuclear test site in conjunction with weapons tests at a time when the long-term effects of radiation on humans were not known.

Army soldiers who took part in at least one of those tests, "Smokey" in 1957, have subsequently suffered an abnormally high number of cases of leukemia. Services' Own Nuclear Race

Interservice rivalry was intense during the 1950s. One of its victims was Dr. J. Robert Oppenheimer, former director of the Manahattan Project.

In the fall of 1951, Oppenheimer took part in Project Vista, a top-secret study of the defense of Western Europe and possible uses of nuclear weapons there. In that capacity, he and other scientists met for several days in France with Eisenhower, who was then supreme Allied commander of the NATO forces.

At a later government hearing, Oppenheimer said that they discussed "anti-air use of atomic weapons, and their use to put out enemy airfields." He added that Eisenhower urged the scientists "to make atomic weapons available."

With the help of Oppenheimer's chapter in the final report on the use of nuclear weapons in a NATO war, the Army was able to win Pentagon and later congressional support for the development of a large number of battlefield nuclear weapons.

Later, according to other testimony at government hearings, Air Force officials were critical of Oppenheimer's assistance to the Army. According to Dr. Herbert York, a physicist who headed the Lawrence Livermore Laboratory in California and later worked at the Pentagon, the Air Force told its civilian scientists not to use Oppenheimer as a consultant and to keep classified information from him because of the help he had given the Army.

The Air Force also forced a reopening of charges that Oppenheimer's earlier associations with American Communist officials and sympathizers represented a "security risk." (Those associations had been investigated prior to the Manhattan Project.) The Eisenhower administration ordered a security hearing in 1954, which resulted in the withdrawal of Oppenheimer's security clearance.

The Navy, too, was eager to join the competition. For example, after learning in 1953 that President Eisenhower was searching for a way to use nuclear weapons in Korea should the truce break, the Navy trained four pilots to fly nuclear-equipped fighter bombers off the USS Lake Champlain.

With specially designed nuclear vaults, the carrier sailed across the Pacific and waited off Korea for the order that never came, according to one of the four pilots, who recently resigned from the Navy as a flag-ranked officer.

In 1958, when Eisenhower again asked about using atomic weapons in case the United States had to defend the islands of Quemoy and Matsu from a Communist Chinese invasion, the three services developed competing plans.

The whole idea was killed when then-Secretary of State Dulles finally got estimates from the AEC that, if executed, the military's plans could kill 8 million Chinese on the mainland.

U.S. nuclear Matador missiles, capable of hitting the mainland, were subsequently stationed on Taiwan.

In the late 1950s, the Strategic Air Command kept some of its large, B36 bombers on airborne alert, a few carrying the Mark 17 hydrogen bomb with an explosive force of 20 megatons, 1,000 times more powerful than the Hiroshima bomb.

Not to be outdone, the Navy flew fighter-bombers armed with nuclear weapons off its carriers in the Pacific toward China, ordering them to veer from the coastline at the last moment so Navy intelligence collectors could read how the Chinese radar reacted.

During the mid-1950s, America's NATO allies wanted more control over the U.S. nuclear weapons within their borders -- only the British had their own weapons then. The Western European allies and the United States discussed and eventually rejected the idea of having multinational crews serve on vessels carrying nuclear weapons. After that, the United States began selling nuclear-capable artillery pieces, missiles and bombs to its allies.

By the end of the decade, West German fighter-bombers carrying American nuclear weapons remained on alert on West German runways. Only a U.S. guard prevented them from being used without the required authorization from a U.S. president.

In the late 1950s, the Eisenhower administration also sold 15 Jupiter intermediate-range missiles, which carried a nuclear warhead, to Turkey. Beginning in 1960, the missiles were installed near the Soviet border, with U.S. troops at those sites controlling the warheads.

Bromley Smith, who worked for the NSC at that time, said recently, "They were there for the purpose of reassuring the Turks that the Russians would not come across their border." Smith said he also had been told that the Jupiters were installed "because we had so many of them that they were coming out of our ears and this was a good place to get rid of them."