North Korea:
Kim actively sought support from Stalin to help North Korea conquer South Korea and unify the 2 halves.
Stalin was initially reluctant to support Kim as he did not want to risk any military confrontation with USA. However, with China there to help support North Korea's efforts and the chance that USA would not be willing to interfere, Stalin changed his mind.
He sent Soviet advisors in to train the NKPD, and sent military equipment such as tanks, guns, artillery and fighter planes.
With rigorous training happening in USSR, and China's military experience from the Chinese Civil War, they now had an advantage over South Korea