What led the North Korean government to invade South Korea?

In 1945 Korea had been divided along the 38th parallel, its industrial north being occupied by the Soviets and the agricultural south by the Americans. Korean leaders wanted a quick reunification, but only on their own terms, and the Communists taking power in the north did not see eye to eye with the nationalists whom the Americans supported in the south. With reunification on hold, in 1948 the Americans and the Soviets respectively recognized the governments in their zone as having authority for the whole country. Soviet and American forces both withdrew, but North Korean forces invaded the south in June 1950 with Stalin's foreknowledge and approval. Within two days President Truman had sent American forces to fight them, acting in the name of the United Nations. The Security Council had voted to resist aggression, and as the Soviets were at that moment boycotting the Council, they could not veto United Nations action.

The conflict escalated into warfare when North military (Korean people Army, KPA) forces-supported by the Soviet union and China crossed the border and advanced into South Korea on 25 June 1975