During the Sino-Japanese War (1894-1895) the Japanese began a move to dominate the peninsula of Korea. With modern weapons and a westernized military, the Japanese easily defeated the Chinese, forcing the Chinese to give up Formosa (modern day Taiwan), and also demanding that China recognize Korea as an independent state. The western powers, who, molded by the era, were all fairly racist, were shocked at Japan's military prowess, modernization, and apparent imperialist desires. Japan did have designs on Korea, and the Russians feared the Japanese might next set their sights on Manchuria, which caused Russia considerable concern.
In the late nineteenth-century, Imperial Russia was extremely interested in north-east Asia. In 1891, Russia began building the Trans-Siberian Railway to connect Moscow and the rest of Western Russia with Vladisvostok. (Vladisvostok, Russia's main Pacific port, means "Ruler of the East.") The best route for the Trans-Siberian Railway was through Manchuria, territory neighboring Korea and which belonged to China. Instead of building along a less favorable route, Russia leased territory from the Chinese on which to build the railway. Completed in 1903, this railway also gave Russia efficient access to the warm- water ports of Dairen and Port Arthur. After Chinese attacks on Russian enterprises in Manchuria during the Boxer Rebellion (1900), the Russian Czar decided to send troops into Manchuria. America's Open Door policy (1899- 1900) in China was partially directed against the expansion of Russian domination in Manchuria. Regardless, Russian and Japanese competition over Manchuria and Korea led to the Russo-Japanese War (1904-1905).
The Japanese fought remarkably well in this major war and defeated the Russian navy. Japan's military might once again shocked the west and soundly embarrassed Russia. US President Teddy Roosevelt stepped in to mediate the Treaty of Portsmouth, which ended the war. The treaty, signed in 1905, allowed Japan to make Korea a protectorate. In 1910 Japan annexed Korea as a formal colony, useful for its agricultural output and mineral deposits.
After the 1917 Communist Revolution in Russia that formed the Soviet Union (USSR), the Russian presence in northeast Asia dwindled somewhat. Japan, on the other hand, became very aggressive, and even sent the Kwantung Army to protect Japanese interests in Manchuria against the Russians. In 1931-1932, the Kwantung Army decided to invade Manchuria of its own volition, setting up a puppet state called "Manchuko."
In 1937, Japan declared war on China, and also began an attempt to "Japanize" Korea by replacing Korean culture with Japanese culture. This included forcing the Koreans to change their names to Japanese ones, making the Koreans practice Shinto (the Japanese religion) and other measures offensive to the Koreans. During World War II, Korea essentially became a labor camp for the Japanese, with Koreans living under armed guard.
The Korean War (1950-1953) had roots in the nineteenth century. Korea was seen as especially valuable to the nations that competed in East Asia because its northern half was blessed with abundant natural resources and sources of hydroelectric power while its southern half was an agricultural breadbasket. Imperial Russia saw south-eastern Siberia, Manchuria, and northern Korea in much the same way the United States saw California and the West in those years. The Russians were expanding into the area, especially Manchuria, and eyed Korea's rich natural resources. As in the American West, the Russians were busy building railroads to transport people and goods across the Siberian wastes to this new frontier. Most importantly, the region had access to warm-water ports which did not freeze in the winter, including Port Arthur and Dairen. Russian leaders had been trying to gain control of a warm-water port for years, but had failed to gain a Mediterranean port. The Soviet Union, which replaced Imperial Russia after the 1917 revolution, also considered Manchuria and the Korean peninsula to be extremely important.
Japan, however, was very worried about the possibility Russia expanding into Korea during the nineteenth-century. And in the early twentieth century, Japan was particularly concerned about the future of Korea because, as a colony, they used it as an agricultural breadbasket to provide their growing urban-industrial classes with rice. While Japan's desire for Korea grew, its options seemed to be limited by the fact that in the face of Japanese aggression, Korea could appeal to the Russians, who controlled neighboring Manchuria, for assistance. In large part, the Japanese fought the Sino-Russian war as an effort to eliminate the Russians as a rival for control of Korea. Incidentally, though Teddy Roosevelt's mediation of the Sino-Russian War in 1905 may have won him a Nobel Peace Prize, it also won the US the long-term hatred of the Japanese.
Because of Japanese domination of Korea in the first half of the twentieth century that culminated with Japan's insulting treatment of Korea during World War II, the Koreans developed a deep hatred of their Japanese overlords, and developed a strong nationalism. Koreans tended to look favorably towards Russia, which had been Japan's recent enemy for influence in the region. Indeed, it was no surprise that many Koreans borrowed from developments in Russia and became Communists themselves. The fact that the most powerful businessmen in Korea were Japanese, while the workers were Korean, was a further reason many Koreans were especially receptive to the Marxist message urging workers to unite and take power. By the end of World War II, much of the Korean population was genuinely pro-Soviet. Therefore, in terms of the Korean War, it is important to recognize that while the Soviet Union had a long history of being interested in Korea's fate, American policymakers saw the country as only an abstract of pawn of symbolic importance in the Cold War.