Korea was fully liberated in 1945 with the birth of the Peoples Republic of Korea (Unified Korea) following the defeat of Japan who had treated Korea like a colony.

The US proceeded, in 1945, to cut the country in half and it was run under a "US Military Government" with the US flying in Syngman Rhee, who between 1905 and 1945 had spent only a few years living in Korea and the most of his time living in United States. So a literal comprador stooge flown in to run America's new colony.

As the communists had been the most ardent fighters against Japanese colonialism they had garnered great respect and began demanding elections for March 1946:

The Korean People’s Republic released political prisoners, organized the distribution of food, and called for national elections as early as March 1946. It announced the confiscation of lands held by the Japanese occupiers and Korean collaborators; an agrarian reform on these and other lands; nationalization of mining, major industries, banking, and transportation; universal suffrage; and a minimum wage and eight-hour day.

from https://www.themilitant.com/2017/8118/811890.html

In response to the overwhelming backing of the communists in Sept 1945, a US military general, declared that the official language of Korea would now be English and Korea would be under US Military control and began putting the Japanese colonizers back into positions of power:

So on Sept. 7, the day before U.S. occupation forces landed on Korean soil, their commander, General MacArthur, decreed that the entire administrative power in Korea south of parallel 38 was under his jurisdiction. The U.S. general warned that, “All persons will obey promptly all my orders and orders issued under my authority. Acts of resistance to the occupying forces or any acts which may disturb public peace and safety will be punished severely.” During the period of military occupation, he said, Korea’s official language would be English.

The U.S. military government refused to acknowledge the Korean People’s Republic and continued enforcing the laws of the hated Japanese colonial administration. The U.S. occupiers even kept in place Tokyo’s officials, including Gov. Gen. Abe Nobuyuki.

from https://www.themilitant.com/2017/8118/811890.html

So given that the Koreans had just fought the Japanese off their soil why should they have allowed the Americans on their soil?

And during the war the DPRK rofl-stomped the south and it took the greatest, most murderous empire this world has ever seen to fight them to a standstill back to the 38th parallel where they had started.

The DPRK didn't have to use child soldiers during that war, but the US-owned South did:

Of the 60, 46 of them were of various ages under 30, including some under 20, and one was a 9-year-old child. The oldest was 51. Forty-eight of the people were described as staff at U.S. military bases in Japan, 12 of whom were minors. The occupations of the other 12 people are unknown, though it is known that six of them were minors.

from https://mainichi.jp/english/articles/20200622/p2a/00m/0na/020000c

And despite the US committing a genocide in Korea by killing 20 percent of the population in the North, they were still unable to beat them.

The bombing was long, leisurely and merciless, even by the assessment of America’s own leaders. “Over a period of three years or so, we killed off — what — 20 percent of the population,” Air Force Gen. Curtis LeMay, head of the Strategic Air Command during the Korean War, told the Office of Air Force History in 1984. Dean Rusk, a supporter of the war and later secretary of state, said the United States bombed “everything that moved in North Korea, every brick standing on top of another.” After running low on urban targets, U.S. bombers destroyed hydroelectric and irrigation dams in the later stages of the war, flooding farmland and destroying crops.

from https://www.washingtonpost.com/opinions/the-us-war-crime-north-korea-wont-forget/2015/03/20/fb525694-ce80-11e4-8c54-ffb5ba6f2f69_story.html

Even committing such crimes against humanity as using biological warfare against them:

• https://www.telegraph.co.uk/news/worldnews/asia/northkorea/7811949/Did-the-US-wage-germ-warfare-in-Korea.html

• https://medium.com/insurge-intelligence/the-long-suppressed-korean-war-report-on-u-s-use-of-biological-weapons-released-at-last-20d83f5cee54

And the only reason Korea isn't unified today is because the USA needs a foothold against China. But the USA will be pushed out of Korea in the next 20 years.

America will pay for its crimes against humanity.

credit to u/JoeysStainlessSteel