I have recently been reading about the cancellation of the USS United States and the subsequent "Revolt of the Admirals." While the main issue of consternation appears to be related to the drastic reductions in funding of the military due to the draw down from the war, there is also a major point of contention having to do with which military branch (the Navy or the Air Force) would be best suited to carrying out long range, strategic nuclear bombardment.
It seems to have been the position of the Truman Administration and the leading military commanders of this time that conventional war had completely changed and that nuclear bombardment would now be the only way to engage an enemy. One amazing (to me anyway) statement coming from Gen. Bradley that no naval landing of ground forces would ever again be conducted. Another point made was that the Navy would be reduced to only transporting cargo.
However, this same administration then found itself waging a conventional war in Korea only a few years later and decided against the use of nuclear weapons from the outset.
So: What changed in the interim?
One major factor changed - the USA lost the Nuclear advantage. 29th august 1949 the USSR successfully tested an atomic bomb.
It would now be a real possibility that if the USA used the A-bomb on North Korea the USSR would retaliate on South Korea. The aggression displayed by the North Koreans in the conflict was in many ways caused by the USSR gaining atomic capability - when Kim il Sung visited Stalin (in may 1949) he was told categorically NOT to invade South Korea. However a year later Kim launched an invasion with Stalins support and blessing. Two major factors had changed - the CCP had gained control of China and the USSR had tested an atomic bomb.
The Chinese situation is also not to be underestimated, though may not have had a massive impact on Us thinking (until Chinese intervention in the conflict the Chinese weren't seen as an international power).
The main reason for a change in policy was the USSR's new nuclear capability, and so a change in USA tactics regarding Nukes was needed
This doesn't really answer your question, but might perhaps revise it somewhat: consider Operation Charioteer.
It was planned at the beginning of the Berlin Blockade (in 1948), after Stalin had ordered the road and rail routes to West Berlin closed "until further notice" (the excuse was that repairs were needed).
The United States determined that they could supply the entire city of West Berlin with food, etc. by air. This was a massive feat of both logistics and courage (with pilots flying for days straight without sleep), not to mention a formidable demonstration of American airlift capacity. (Made all the more impressive over the winter, when the aircraft supplied heating coal as well as food.)
After the airlift began, however, there was a great deal of worry on the American side about whether the Russians would close the air corridor as well. The Red Air Force wouldn't even have to shoot down American transports... all they would have to do would be to stage large-scale "military exercises" in the three (treaty-delineated) air corridors from West Germany to West Berlin: to supply the city, American transport aircraft were stacked every few minutes--a plane landed every 30 seconds at the peak of the airlift-- and while individual MiGs would buzz the incoming transports, a full-scale Soviet air presence in this tightly-stacked airspace would have caused chaos (not to mention likely mid-air collisions).
So, to dissuade Stalin from this option, the Joint Chiefs of the Joint Intelligence Group drew up a plan, codenamed "Charioteer," which called for--in the event that the Soviets closed the air corridor--the immediate dropping of 133 atom bombs on 70 Soviet cities and industrial centers (with eight on Moscow and seven on Leningrad) with an estimated destruction of 40 percent of all Soviet industry and killing nearly seven million Soviet citizens. This plan was then "leaked" to the Soviets (by what means, unfortunately, I am not aware: any Diplomatic/Military/Cold War historians out there to help me out?)
The upshot: Stalin never ordered the air corridor closed.
But the big picture (as it applies to your question):
In 1948, the go-to option for the Joint Chiefs to counter Stalin's aggressive moves was logistical (not military). But...
in 1948, the Joint Chiefs were definitely willing to include an atomic first strike against the USSR as part of their military and/or diplomatic contingency plans, if the logistical option was taken off the table.
Everything would change in 1949, , of course, when the Soviets detonated their first atomic bomb.
What purpose would using an atomic bomb in Korea serve?
First, the North Koreans were able to achieve such stunning victories in the beginning because they caught the US with their pants down. The US had pulled out of Korea in 1948, leaving a handful of advisors to help the ROK military. They refused to seriously arm the ROK military out of fear that Syngman Rhee would use them to invade the North. After the blitzkrieg attack from the North, the South, with US/UN support, was able to stop the advance at the Pusan Perimeter and to reverse their losses with the Inchon landing and the breakout. By mid October 1950, Pyongyang had fallen and the war was almost over - except the Chinese intervened, the allied forces were pushed south again before coming back north and stabilizing along the 38th parallel.
Using nuclear weapons wouldn't have helped. First, the US bombed the hell out of North Korea with conventional weapons. They bombed cities, towns, villages, farms, factories, and finally, even dams. Anything that moved, the US Air Force dropped a bomb on it during the Korean War. There was no need to use nuclear weapons. Second, the US had bases in Japan and China had bases in Manchuria, neither of which were attacked. If the US had used nuclear weapons on China (which was the argument in the first place, bombing Chinese bases north of the Yalu), it could have widened the scope of the conflict and brought Japan into it, something neither side wanted. Finally, from mid 1951 until the end of the war, the two sides were negotiating for peace. Escalating the war by using nuclear weapons was not in anyone's interest.
As far as the US decision, it was not decided at the outset. MacArthur made rumblings about using nuclear weapons and Truman announced in late 1950 that the US was considering using nuclear weapons in Korea and MacArthur submitted a list of targets he would consider using nuclear weapons against. There is a lot of confusion on the issue. MacArthur submitted targets and Truman later claimed that MacArthur wanted to use nuclear weapons. MacArthur claims he never discussed using nuclear weapons and Truman then said that it was his opinion that MacArthur wanted to use them. The facts are that MacArthur did have a list of targets that was submitted to the President and the National Security Council, and Truman did say that the US was considering using them and that the field commander had the authority to decide if and when to use them. If either MacArthur or Truman really wanted to use them or planned to use them - who knows. The two of them didn't have a good relationship and what they said publicly and thought privately didn't always line up.
/u/Rojav mentions that the US lost the nuclear advantage, but this is inaccurate. The Soviets had achieved a nuclear weapon in August of '49, but it wasn't until October of '51 that they had a bomb that could be delivered by air. At that point, the US military had over 300 usable bombs. Escalation to full blown nuclear war was not a realistic scenario. Further, the Soviet's concern was with Europe, not Asia. It was the Secretary of State's speech in January 1950 that left Korea out of the US defensive perimeter that was the tipping point for Stalin in giving approval to Kim, as well as the communist victory in China and the support that they could offer. In the end, the Chinese bore the brunt of the war with the Russians playing a very minor role in it.