Very good summary of the origin's of Korea's division and the Korean war.

Some extra information to add on:

A 1946 opinion poll appearing in Korea's Dong-A Ilbo newspaper showed a majority of respondents favoring socialism as their preferred system, and less than 15% supporting capitalism. Together, socialism and communism received 77% support from survey respondents. (Source)

In 1950, when the Korean People's Army had taken control of Seoul, the CIA wrote that "over 50% of Seoul's students are actively aiding the Communist invaders, with many voluntarily enlisting in the Northern Army" and that among Seoul's population, "the working class generally supports the Northern Koreans" adding that the streets of Seoul were "crowded [...] especially with youths engaging in Communist demonstrations." (Source)

Meanwhile, here is the person backed by the US to run the southern regime:

Syngman Rhee who was the first president of the Republic of Korea, is responsible for the killing of 30,000 Jeju islanders between 1947 and 1954 (during the Jeju April 3rd Massacre). Secondly, he is responsible for the massacres of one million civilians during the Korean War (1950~1953), and finally, he illegally amended the Constitution in 1954, aiming for long-term seizure of power and initiated fraudulent election in 1960. ("Letter from 252 South Korean NGOs against Syngman Rhee Day")

A 1948 CIA report wrote regarding Rhee that "there is every prospect that Rhee's accession to power will be followed by intra-party cleavages and by the ruthless suppression of all non-Rhee Rightist, Moderate, and Leftist opposition," characterizing Rhee as an "imported expatriate politician" and "extreme rightist" and demagogue "bent on autocratic rule", who would be an "unpopular" figure who would play into communist propaganda due to his extreme rightist orientation, and stating that the U.S. throwing their full support behind him could potentially be "a source of future embarrassment to US policy in the Far East." (Source)