This is a really fascinating topic. And coincidentally, it was the subject of my Master's thesis over a decade ago! I'd just like to add my thoughts to what's already been written.

The outbreak of the Korean War came as an immense shock to American strategic calculations and the conflict's early phase was characterised by fears that the DPRK invasion—backed, it was assumed in Washington, by the USSR and PRC—presaged a much wider communist assault on the West. The atomic bomb was brought into discussions on the very first day of the war and remained a factor for the next three years. Truman found himself in a paradoxical situation: horrified by the effects of the atomic bomb and at the same time fully aware of its importance in America's stand against communism. Truman placed himself under a mental strain that would last until he absolved himself of responsibility for nuclear decision-making by refusing to run for the Presidency in 1952.

When, on November 30 1950, Truman responded to the goading of the press with the statement that nuclear weapons had always been considered as tools to aid the war effort, the world exploded in uproar. This period was vital in the Truman administration's 'nuclear education', highlighting the extreme response to the merest mention of the A-bomb. Also of significance were the accusations of 'appeasement' levelled at the administration when they chose not to use the full power of the atomic arsenal against the Chinese armies. Fully aware of the weight of such accusations—redolent as they were of Chamberlain, Munich, and the Nazis—Truman and his colleagues formulated plans to offload the blame for the strategy onto British Prime Minister Clement Attlee. Appeasement accusations form an underestimated influence on Truman and the administration. The 'offloading' was not enacted and this strand of criticism would continue to provoke debate throughout the Korean War.

The events of early December1950 were followed by a three month period where further accusations of appeasement, the influence of foreign opinion, and increasing awareness of the subtleties of atomic diplomacy all contributed to the picture of non-use. And as regards Douglas MacArthur, rather than simply requesting atomic bombs for use in Korea because of his aggressive personality—which was undoubtedly a contributing factor—he was also provoked by defence intellectuals providing pro-bomb intelligence and the demands of a JCS desperate for his opinions on the situation. In sum, the events of this period demonstrated that non-use was never off the table and that extreme positions—such as those expressed by Stuart Symington in NSC-100—were also ruled out. In many ways this refutes the older findings of scholars such as Daniel Calingaert that in Korea U.S. policy refused to allow outside restraints to limit its freedom to atomic weapons. The U.S. found itself restrained on a fundamental level by foreign powers, shackles that it would however attempt to throw off in the coming months.

However, so terrifying was the situation in April 1951—with intelligence indicating that the PRC and USSR were planning massive air assaults on UN forces in Korea—that Truman overcame his horror of making any commitments to use the bomb. In deploying weapons to Guam in April 1951, Truman took a step that had not been taken since 1945. Despite this singular move towards nuclear use, Truman remained conflicted about the atomic bomb. He remained determined never to make that final order for use, but the deployment—and his willingness to dismiss the overly aggressive and confrontational MacArthur— demonstrated a new sophistication in how atomic weapons might be used. There was also a subtle, but remarkable, change in the British position on atomic use. Foreign Secretary Herbert Morrison gave his tacit agreement to nuclear use under limited circumstances. This demonstrates that, as time went on, the Truman administration began to mould the opinion of its allies to its way of thinking, rather than vice versa.

From July 1951 to January 1953, the evolving theory of tactical nuclear weapons use and the non-use debate clashed. Prior to the 'tactical revolution' engendered by the Korean War, nuclear weapons had broadly been seen—with some notable exceptions—as purely strategic instruments. Korea demonstrated the dangers of potential strategic use and therefore pushed planners towards the search for battlefield uses. Notions of the tactical applications of the A-bomb provoked great debate in Congress and the media, with key figures from the administration stoking the fire. By this stage in the war, however, the time for nuclear weapons to decisively influence the course of events had passed. Korea had become a static war and in such an environment there could be little, if any, justification for employing weapons as powerful as the atomic bomb.

Fear of World War III resulting from the conflict in Korea was always an issue for insiders and outsiders alike. From the earliest days where it was suspected that Korea was the first of a series of communist moves against the West, to the final days when recalcitrance at the negotiation table was viewed as a stalling tactic prior to a massive attack that fear of provoking World War III never subsided. Hence, the terrifying prospect of a global war between East and West served to exert a restraining influence.

The influence of accusations of appeasement—with which political opponents and the general public alike bombarded the administration and that were most notable in their effect during the 'consideration' débâcle of December 1950—had a noticeable impact on key actors such as Truman and Acheson. The accusations caused restraint due to the knowledge within the administration that nuclear use would lead to global war. Cries of 'appeasement!' served not to push Truman and his colleagues towards use, but served to push them away from nuclear use by making it clear that they had to limit the war even though still determined to confront the 'aggressor'.

World opinion also served to create a barrier against use. This was expressed in many ways. From a practical point of view, the United States could not afford to lose the goodwill of the UK and France if it wished to base its advance nuclear strike force in Europe. The need to retain alliances in order to have adequate striking power against the Soviet Union in the event of a major war was paramount in the minds of many.

Likewise, the opinions of other nations helped to reinforce the moral and ethical arguments against use. The issue of race was brought to the table on more than one occasion. The majority of Asian nations perceived the A-bomb as a fundamentally racist weapon, one that white powers only ever used on non-white people. If the bomb were used against Korea or China, the United States would lose the support of allied and non-aligned nations in its fight to enact the policy of containment. The question of race was reflected in official documentation and public discourse: some more extreme U.S. citizens viewed Korea as nothing more than a long awaited 'race war' and heartily encouraged the use of the atomic bomb against 'the yellow race'.

Hope this helps broaden things out a little.

Malcolm

Greetings! This question is most interesting, and it's great to see a fellow contributor who takes a keen interest in all the WW2 nuclear-related posts on here. I will attempt to give my tuppence on the matter, though do keep in mind that (as far as I'm aware, though I may be completely wrong), that this still remains a highly debated topic in the historiographical communities, and thus there is no "end-all" answer (as there almost never is in History). So let us begin.

First however, a simple quote from a high-school textbook which we shall analyse and thoroughly 'test':

For a time, Truman and his administration considered MacArthur's call for the use of nuclear weapons against China. In the end, after strong opposition from Britain's Labour prime minister Clement Attlee and worries that such action might end in a nuclear war with the Soviet Union, this option was rejected.^(1)

The whole affair with Truman and nuclear power started well before Korea was even an active battlefield, when he authorised the first (and so far only/last) deployment of nuclear weapons against Hiroshima and Nagasaki during the Second World War. He therefore, of all people, knew the conflict-ending power of the atom; so it was hence an entirely valid prediction that America might bring this potent new weapon to bear against the North Korean and Chinese forces in 1950. As historian Nathan A. Jennings points out:

"Reflecting historical irony, the single most influential person in the history of nuclear non-use is the man who also authorized the first use: Harry Truman."^(2)

In terms of Truman's own atomic policy, he had actively ordered the movement of nine Mark 4 Nuclear bombs into military possession in April 1951.^(3) Although these bombs only went as far as Strategic Air Command's bases at Guam and Okinawa, it does certainly support the theory that Truman had plans (if unlikely ones) for a potential strike in Asia (note: this move was so secretive that not even MacArthur was aware of the deployment).^(4) On the other hand, Truman knew the various geopolitical and moral consequences of deploying an atomic weapon in Korea, and his advisers pressed against such usage.

Up until the Chinese People's Volunteer Army (PVA) poured across the Yalu river into North Korea and the war in November 1950, there had been serious calls from both the Pentagon and military for the deployment of America's atomic power. After the PRC's intervention however, the situation changed drastically. Many historians writing on the subject (and most likely OP themselves) point out Truman's 'blunder' during a November 30th Press conference, in which the following remark was made:

" 'The President [Truman]: We will take whatever steps are necessary to meet the military situation, just as we always have.'

Question: 'Will that include the atomic bomb?'

The President: 'That includes every weapon that we have.'

Q. 'Mr. President, you said ‘every weapon that we have.’ Does that mean that there is active consideration of the use of the atomic bomb?'

The President: **'There has always been active consideration of its use.'**^(5)

This remark threw the entire government into chaos, and the White House had to issue its own note later that same day noting that the President retained supreme authority over the use of atomic weapons, and that Truman's 'ill-considered' remarks (as historian Rosemary J. Foot calls it) changed nothing.^(5) Even so, Truman unwittingly set off alarm bells across the pond in Westminster, where the House of Commons had been engaged in a two-day debate on the situation in Korea. Historian Rosemary Foot on the scene in the houses of Parliament:

"[C]oncern turned into alarm when Truman's comments on the possible use of atomic weapons was reported. Seventy-six Labour MPs signed a statement decrying the American president's proposal and announcing that they would withdraw their support from the British government if it went along with a U.S decision to expand the war into China. Given the prospects of a wider Far Eastern conflict developing within a matter of weeks, shadow and cabinet ministers urged that Britain be represented at the highest level in Washington. [Prime Minister] Attlee quickly responded to the call and made the decision to go to Washington at an emergency cabinet meeting the next day."^(6)

The British vehemently opposed the use of American weapons without prior consultation and approval from London, a condition which Truman compromised on with Attlee by informing him that should the decision be made, Britain would be the first country to find out.^(7) Alongside endangering the "Special Relationship", Truman also had to deal with the fact that technically America was commanding a UN Task-force in Korea, and that deployment of an atomic bomb without the UN's involvement would shatter international unity and, perhaps more importantly, the reputation of America. Before moving on with why Truman and his administration finally chose not to use nuclear weaponry, I would like to drop in this somewhat scathing remark on the role of British opposition in Truman's decision (from a historian whose bias against the Labour party is most apparent in the source article):

"The US government’s decision not to use nuclear weapons had apparently nothing to do with Attlee’s representations. What we can surmise is that if the Americans had used nuclear weapons, the Labour government, with whatever private reservations, would have supported this, just as it had the bombing of Hiroshima and Nagasaki. Attlee’s concern appeared to be not the loss of Asian lives (there was no objection to the massive scale of conventional bombing in Korea), so much as the potential loss of British lives if the Soviet Union were to retaliate."^(8)

It is this Soviet retaliation which we then turn to next. Truman had, since his Truman Doctrine in 1947, pursued a policy of containment, in which the borders of Communism remain in their current state and expand no further. With this consideration in mind, he essentially desired a return to the status quo ante bellum*,* in which the North Koreans were repulsed from South Korea, but not necessarily eliminated as a nation.^(9) He feared, alongside advisers such as Dean Acheson and the State Department, that the Soviets would fully enter into the conflict if America utilised its atomic power. Whether this meant increased military support to North Korea and the PRC, or more alarmingly, the possibility of conventional retaliation in Europe, was a possibility Truman did not wish to turn into reality.

So there we have it, a curious turn of events and strategic outlooks if ever there was one. In the end, Truman's administration chose not to use nuclear weapons against China (despite having clearly considered plans to do so) for a litany of geopolitical, diplomatic, and anti-escalatory reasons. As the situation in Korea worsened, the State Department's officials argued on the subject of deploying nuclear weapons that...

"the probable costs of doing so - measured in terms of shattered UN unity, decreased respect in Asia, and possible war with China - far outweighed any possible military gains."^(10)

That quote in of itself sums up the situation quite nicely, and hope that this response as a whole is somewhat enlightening towards your question. The sources attached below should also serve as excellent further reading guides, as they detail in-depth the "journey" of Truman's administration from pro-nuclear deployment to anti-nuclear deployment, as well as how this inaction led to a cornerstone of US nuclear deterrence policy throughout the Cold War. Happy reading!

This is an important and difficult historical question, and there are divisions between historians and political scientists as to how to answer it. The trickiness is that this is a period in which the US had a virtual nuclear monopoly; the Soviets had only detonated a single bomb in late 1949, and had nothing like an arsenal, and had no means of projecting power with it, and so on, so it could not be considered a true nuclear deterrence situation. So why did the US not use nukes, especially when some members of its military — notably MacArthur — thought it was a swell idea? What was the point of this new weapon, and the nuclear monopoly, if it couldn't be used?

The historical dispute comes down between two groups that one could call the "rationalists" and the "emotionalists." The rationalists argue that the bomb was not used because of very strictly logical reasons, notably the fears of escalation (even if only conventional), the fears of negative public and international opinion (the idea of the US once again using nukes on Asian peoples was not considered a "good look"), and the fears of both friendly fire and the waning value of nukes (if you use nukes and they don't end a war, does that encourage their future use and devalue them?).

The "emotionalists" (I am just making up these categories) basically say that the non-use of nuclear weapons then was not about rational calculations, but a sort of visceral abhorrence to the further use of nuclear weapons. In this model, a "nuclear taboo" was emerging that was rooted in a moral and emotional dislike of nuclear weapons. For the Korean War, the "taboo" was not fully universal in any strong way (it never becomes totally universal), but was emerging and became somewhat cemented because of the non-use even when non-use was not compelled by a deterrence situation. The evidence for this approach is specifically in the personage of the only person who had the power to order such use, Harry Truman, who in the period after Hiroshima and Nagasaki became increasingly uneasy with the idea of nuclear weapons use and associated them primarily with the slaughter of innocent civilians.

For non-use in Korean War, you can find support for both of these arguments, which is perhaps a sign that they are not mutually exclusive. The Joint Chiefs of Staff and other military commanders did discuss the possibility of nuclear weapons use, but their memos make it clear they were very hesitant for the "rationalist" reasons I've given above. It was just not clear that using nukes would actually make a big difference tactically (it wouldn't obviously lead to an unambiguous or quick military victory), and the potential political consequences (the Korean War, remember, was a UN police action, and the US was feeling more pressed than ever to make sure it had its allies in line with it as it confronted a newly-nuclear Soviet Union) would be high. It could also have a hugely escalatory effect, bringing in more Chinese or direct Soviet support of the North Korean troops that could make the war much harder to win, and potentially escalate into a more general war.

On the other hand, one can clearly see that Truman never gave the idea a lot of thought either way, and that he had by that time built up a severe revulsion to the use of the weapons, and a distrust of military enthusiasm for them. He quipped once about their possible use, and deployed a tiny number of bombs to the theatre, but it is clear he never had any interest in their use.

Had Truman pushed for their use, the military probably would have used them. Had the military pushed for them to be used, Truman probably would have refused on emotional grounds. But neither pushed (except MacArthur, which Truman saw as insubordination).

This brings up the interesting counterfactual: what if Truman wasn't the president, then? If, say, Eisenhower was president in 1950, would nuclear weapons have been used in Korea? Other scholars have concluded (and I concur with them) that the likelihood is very high. Eisenhower did not have Truman's moral hesitations about using or threatening to use the bomb (he used nuclear blackmail against China during the Taiwan Strait Crisis, for example), and thought tactical nuclear use was an acceptable option (using the bombs only against troops) early in his presidential career (his attitudes cooled over the course of the 1950s). With a different president, you possibly get a different result; this is one of Truman's most under-sung legacies, is that he effectively established a norm of non-use of nuclear weapons after World War II, even though he could have probably pushed for the contrary if he wanted to.

Anyway, this is just an overview. My favorite source on this is a chapter in Nina Tannenwald's The Nuclear Taboo, which as the title indicates is in the "emotionalist" camp for explanation, though she explores other reasons as well. For a contrary, "rationalist," view, see T.V. Paul, The Tradition of Non-Use of Nuclear Weapons. I find Paul's arguments less compelling as a historian who has looked quite closely at Truman in particular; I do not see "rationalist" justifications for any of his actions (that is just not how Truman operated). For more on Truman's own apparent about-face in nuclear attitudes after Hiroshima and Nagasaki, and how one might resolve the apparent contradiction of them, one might also look at my piece published in early 2020 about Truman, the Kyoto decision, and his changing attitudes on the bomb.