re: A bit of the history
The North Koreans with the help of the CCCP began preparing for the invasion well before the American "left" leaving only a small advisory force of Americans in South Korea. In 1949 guerrilla teams were sent from N. Korea into the South there defeat signeled to Kim that only a conventional attack would take over the country.
Remember one reason the Americans left is that the Soviets left.
The KMAG (American Advisors) consisted of around 500 men at the time of North's conventional attack.