North Korea had the backing of the Soviet Union and were propped up by them.
“Despites its denials at the time,
the Soviet Union was intimately involved in the Korean War. The contribution made by the Soviets was vital. They provided diplomatic support, strategic and grand tactical planning, including the planning of the invasion of South Korea, and essential logistical support. (
Source)”.
“Though not officially a belligerent during the
Korean War (1950–1953), the
Soviet Union played a significant, covert role in the conflict.
It provided material and medical services, as well as Soviet pilots and aircraft, most notably MiG-15 fighter jets, to aid the North Korean-Chinese forces against the South Korean-United Nations Forces. Joseph Stalin had final decision-making power and several times demanded North Korea postpone action, until he and
Mao Zedong both gave their final approval in spring 1950 (
Wikipedia Source)”.