Lazy NATOpedia understanding of history.

You start the story in the middle because you've been taught that way. Korea was occupied by the Japanese from 1910 to 1945. Kim Il Sung gained fame as an anti-Japanese guerilla fighter. Korea was sliced in half after WW2. A soviet occupied half, and a US-occupied half. Here's how the Koreans were greeted respectively by the Americans and the Soviets, who were temporary allies in WW2.

The Americans and the Soviets naturally couldn't agree on a trusteeship because the Americans wanted to force private enterprise and fascist Japanese collaborators onto the Koreans and the Soviets wanted Korea to be governed by the Korean working class (or, as imperialists call it, totalitarianism)

[Content warning: various atrocities]

The US instituted a US-backed military dictatorship in 1945, enabled that military dictatorship to begin executing socialists, anarchists, and Communists, placed people who had collaborated with the Japanese into positions of power, etc. The US-led South provoked the North with artillery strikes and armed incursions for 2 years leading up to the Korean war. When the "North" Koreans finally responded to this American occupation, the "Korean War" is said to have started (according to the Americans, who always start the story late), the American air force leveled every single city in the North, did not care about civilian casualties on either side, burned villages with flamethrowers, committed germ warfare, bombed schools, hospitals, dams, etc. And infantry committed sexual assault and executions on civilians. The fascist maniac General Douglas MacArthur (who Americans still worship as a "good man" in much the same fashion Americans accuse "North" Koreans of being brainwashed) wanted to nuke the entire 38th parallel but was thankfully stopped. After LOSING MISERABLY (as Americans always do in these kinds of wars, all they can do is get a high kill/death ratio by murdering civilians, they never actually achieve their strategic objectives) they continued to maintain a military occupation of the South (against the wishes of most South Koreans) LONG after the Soviets and PRC left North Korea, and they continue starving the North Koreans with sanctions for 70 years so they cannot achieve economic independence, and they blame the consequences of these sanctions on the "inferiority of socialism" while injecting billions of into the South Korean bourgeois system to keep it afloat and turn it into a giant theme park for US imperialist soldiers and sleezy businessmen to commit sexual assault on local women. Fuck off.

(you will naturally call this history "biased" because it tells the truth instead of devolving into lazy "both sides are equally bad" algorithmic assessment or the even stupider "actually the koreans themselves invaded their own country" narrative)

Burger Reich troops should GTFO of South Korea btw. They're not wanted there. They're fascist thieves and unwelcome occupiers and regularly get drunk and commit atrocities against locals