Why would Russia support NK in the war for such a small plot of land? Was it to spread communism? And did we really stop the spread of communism through helping SK win the war?
The USSR wanted buffer states around it, to protect from possible Invasion. (As seen by the warsaw pact in the west)
If all of the korean peninsula (which has quite a large border with Russia) was governed by a capitalist, US friendly country, the USSR would have issue with that.
Also it was PRC that contributed the most to the korean peninsula in terms of manpower. The MiG-15 (USSR made) was seen quite often though. So no doubt the USSR funded and equipped the NK.
And yes I suppose communism was halted at that parallel in the korean peninsula.
It's important that we define "support" here. By way of example, the North already existed as a bit of a client state of the USSR's owing to the latter's occupation of and establishment of the former at the end of the second world war.
So roundabouts 1949 Kim Il-Sung begins pushing Stalin to back his play to reunite the Korean peninsula under a communist banner. Stalin is dismissive of this idea -- the Americans still have troops in South Korea and the notion of starting a war with the USA isn't appealing to him.
But a great deal changes by 1950. American troops withdraw, as planned, from Korea and -- and there is some debate about how important this is -- Dean Atchison, then US Secretary of State, made a speech outlining a "defensive perimeter" in Asia which excluded Korea.
In the first place, the defeat and the disarmament of Japan has placed upon the United States the necessity of assuming the military defense of Japan so long as that is required, both in the interest of our security and in the interests of the security of the entire Pacific area and, in all honor, in the interest of Japanese security. We have American—and there are Australia—troops in Japan. I am not in a position to speak for the Australians, but I can assure you that there is not intention of any sort of abandoning or weakening the defenses of Japan and that whatever arrangements are to be made either through permanent settlement or otherwise, that defense must and shall be maintained.
The defensive perimeter runs along the Aleutians to Japan and then goes to the Ryukyus. We hold important defense positions in the Ryukyu Islands, and those we will continue to hold. In the interest of the population of the Ryukyu Islands, we will at an appropriate time offer to hold these islands under trusteeship of the United Nations. But they are essential parts of the defensive perimeter of the Pacific, and they must and will be held.
The defensive perimeter runs from the Ryukyus to the Philippine Islands. Our relations, our defensive relations with the Philippines are contained in agreements between us. Those agreements are being loyally carried out and will be loyally carried out. Both peoples have learned by bitter experience the vital connections between our mutual defense requirements. We are in no doubt about that, and it is hardly necessary for me to say an attack on the Philippines could not and would not be tolerated by the United States. But I hasten to add that no one perceives the imminence of any such attack.
Acheson says that on January 12, 1950 and three months later Stalin gave Kim the go-ahead to invade (conditional upon Chinese backing as well).