Why did Stalin give North Korea permission to invade South Korea?

Stalin gave North Korea permission to invade South Korea in hopes to unify the Koreas under a single communist government, the North launched a surprise invasion of South Korea on June 25, 1950. In the following days, the UN security council condemned the attack and demanded an immediate withdrawal. And because of that, Kim II Sung had hopes to join the two Koreas into one communist nation in order to spread communism.