T. R. Fehrenbach explains, in This Kind of War, why there were multiple Korean wars before the Korean War:
Korea, or Chosun, is a peninsula, 575 miles in length, averaging 150 miles across. It resembles in outline the state of Florida, though bigger. Along its eastern coast a giant chain of mountains thrusts violently upward; the west coast is flat and muddy, marked by estuaries and indentations. Inland the country is a series of hills, broad valleys, lowlands, and terraced rice paddies. Its rivers run south and west, and they are broad and deep.
It is a country of hills and valleys, and few roads. Most of Korea is, and always has been, remote from the world.
Chosun is a poor country, exporting only a little rice. But its population density is exceeded in Asia only by parts of India.
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Neither China, nor Russia, nor whatever power is dominant in the Islands of the Rising Sun, dares ignore Korea. It is, has been, and will always be either a bridge to the Asian continent, or a stepping-stone to the islands, depending on where power is ascendant.
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Manchuria is the richest area in all East Asia, with iron ores, coal, water power, food, and timber, and whoever owns Manchuria, to be secure, must also own Chosun.
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It is the nature of peoples to see the ancient foes, and to ignore those newly arising. Japan defeated Russia with the moral and material aid of Great Britain and America, who had watched the Russian advance to the Pacific with unconcealed dread. Japan, with far greater ambitions than the rotting Empire of the Bear had ever entertained, now was the dominant power in East Asia, and America and Britain applauded.
They did not sense that, in time, Japan would overthrow the old order completely.
“Manchuria is the richest area in all East Asia, with iron ores, coal, water power, food, and timber, and whoever owns Manchuria, to be secure, must also own Chosun.”
The invasion of 1950, as approved by Stalin, was to outflank the Red Chinese, in order to redeem and reestablish Imperial Russian authority in the area.