Beginning in March 1949 Kim Il Sung lobbied Stalin for approval of and matériel support for a military attack on the ROK. Stalin initially demurred. At the end of January 1950, with the Communists having won the civil war on mainland China, with Mao in Moscow negotiating a military alliance with the Soviet Union, and with support for the ROK in the United States appearing less than firm, he changed his mind. Over the next several months, Stalin approved the shipment to North Korea of heavy arms, including tanks, thus giving the DPRK a clear military advantage over the ROK. North Korea was also strengthened by the return of tens of thousands of Korean nationals who had fought on the Communist side in China. In meetings with Kim in Moscow in early April, Stalin explicitly approved a North Korean attack on South Korea, provided Mao also gave his blessing. Although he believed that the United States would not intervene, especially if the North Koreans won a speedy victory, he made it clear that, if Kim ran into difficulty with the Americans, he would have to depend as a counter on direct Chinese, not Soviet, intervention. When in mid-May Mao endorsed Kim’s proposal for an early attack on the ROK, the plans proceeded to their final stage.

Even with the intervention of U.S. troops in July, the DPRK nearly drove the enemy out of Korea. By early August forces fighting under the UN banner were squeezed into the Pusan perimeter, on the southeastern corner of the peninsula. At the end of the month DPRK forces launched an offensive that over the next two weeks inflicted more enemy casualties than in any other comparable period during the war.

Yet UN troops now outnumbered their opponents and, on 15 September, General MacArthur launched a counteroffensive at Inchon, the port for Seoul. By month’s end UN forces had broken out of the Pusan perimeter and retaken Seoul. DPRK forces were in headlong retreat northward and the United States had altered its objective from reestablishing the thirty-eighth parallel to destroying the enemy and reuniting the peninsula under a friendly government.

Source

More here. Some of the documents mentioned can be found in these archives.

This was a deliberate communist attempt to conquer South Korea with the approval of both Stalin and Mao. Without military intervention, particularly from the U.S., it would have happened, and all of Korea would be like the north is now.