Hello! I love this subreddit and this is my first time posting. I'm reading a book that was reccommended here about North Korea called "Under the Loving care of the Fatherly Leader". In one part I just read, the author was describing how Pyongyang started militarizing its forces in the late 1940s to prepare for a Korean War and they were aided by the Soviets, while the South Koreans were hobbled and held back by the US. Why didn't the US hold back South Korean forces?
I can't answer your question in full, but a few points to bear in mind: not only did the Soviets support the North Korean military, but thousands of troops who were veterans of the Chinese Civil War came back to North Korea in 1948-1950. Many other Koreans came to North Korea from Yanbian in 1946-1947, where although they did not take part in any of the major campaigns of the Chinese Civil War, they did gain valuable organizational and military experience in "bandit annihilation" and other military-political mobilization campaigns. None of, or very few, of these veterans returned to the South, giving the North an advantage at least in terms of experience (and some weaponry).
Moreover, the ROK and the United States were busy staving off huge, violent insurrections and soldier-led rebellions in the deep south (Cheju, Yosu, and Sunchon) in 1948-1949. This proved to be particularly taxing on the ROK government and its armed forces. The North faced no such internal turmoil. The worst documented case of political turmoil in the North came in November 1945 in Sinuiju, but this was relatively small (a student protest, at most dozens dead) and was not prolonged.
At the same time, many US leaders were deeply ambivalent about Syngman Rhee and preferred to investment in other strategic objectives in Asia (Japan, China, Taiwan, Vietnam) over Korea, at least until South Korea satisfied some basic political demands of the US.