Hey, a question in my field that I feel qualified to answer!

Koreans have only very recently enjoyed an independent status as a nation in East Asia, and it was after a considerable struggle against foreign occupying powers and subsequent division of the nation. Since at least the Han Dynasty, Korea was subject to the Chinese tribute system and was informally subjected to Chinese rule. Nominally, Korea was an autonomous region with its own king. In reality however, Korea was still subservient to the Chinese, with the king directly dependent on the Chinese emperor. In fact, even their writing system still widely used Chinese characters even after the creation of a formal Korean character system in the mid 15th century. Because of this common heritage and vast amounts of overlap in the two cultures, Korea and China actually had quite amicable relations until relatively recently. Where Chinese relations with Korea begin to deteriorate is around the end of the 19th century, with China's defeat at the hands of the Japanese in the First Sino-Japanese War. The Japanese-led reforms in Korea after the war, known as the Kabo Reforms, were aimed at separating Korea from Chinese influence. They forced government documents to be written in Korea, and declared Korean independence from China, amongst other things. Following this period Korea enjoyed trade with Japan, China, and Russia and was no longer entirely dependent on China. From 1910 to 1945 Korea was annexed and occupied by Japan, which I will go over later when discussing Japanese-Korean relations. Following Japan's loss in WWII and subsequent loss of its empire, Korea was immediately occupied quite similarly to Germany following WWII. South Korea was occupied by the Americans, and North Korea occupied by the Soviets, as demarcated by the 38th Parallel. However, on June 25th, 1950, South Korea was invaded by the Russian and Chinese-supported North. Though a counter-offensive by the UN eventually pushed the border back to the 38th Parallel, the damage to foreign relations was already done. In a similar way that Koreans across the border harbor resentment toward one another for the Korean War, many in South Korea do not forgive the Chinese for supporting the invasion by the North, and their continued support for the DPRK. It wasn't until the enactment of the South Korean Nordpolitik in the 1980's that Soviet relations were finally thawed.

Whereas Chinese relations are cold due to its secondary support of conflict in Korea, Japanese relations are directly related to its violent conflict with Korea. Japan's long history of conflict with Korea begins with Hideyoshi's attempt to conquer China by going through Korea in the late 16th century. He was eventually defeated by Chinese forces with the final withdrawal occurring as a result of his death. It wasn't until the late 19th century that Japan again began to encroach on the Korean peninsula. Japan forced Korea to open itself to trade with Japan under the Japan-Korea Treaty of 1876 (one of the infamous unequal treaties forced onto developing nations by the imperial powers at the time.) At the end of the First Sino-Japanese War, Japan forced China to accept Korean independence under the Treaty of Shimonoseki. Then, 15 years later Japan annexed Korea in 1910 and conducted an occupation until its defeat in WWII in 1945. During this occupation anti-Korean sentiments ran rampant in Japan and amongst the Japanese who saw the Koreans as lowly, unintelligent laborers much like those in occupied Taiwan. (from Yom Sang-seop's "On the Eve of the Uprising") Also, toward the end of the occupation, during the Second Sino-Japanese War, the Japanese military sponsored kidnappings of Korean women to serve as "comfort women" for Japanese soldiers. This was equivalent to forced sex trafficking, and has been a major source of conflict and embarrassment for the Japanese in the 21st century. A quick google search of "comfort women" will net you hundreds of results on the topic. Perhaps even more atrocious was the Japanese medical experimentation Unit 731 (NSFW) which performed many grotesque experiments and operations on still living patients, many of whom were POWs, or civilians kidnapped from invaded areas.

Post-WWII sentiments have been warming up toward the Japanese on all fronts, however, especially given their demilitarization following WW2.

In regards to relating this conflict with Europe, one of the most common parallels drawn is the treatment of war criminals in the aftermath of the war. Whereas the Nuremburg trials saw significant convictions and an in depth investigation into "crimes against humanity," as well as the death of the Nazi leader, the Tokyo War Crimes Trial saw very few convictions and the emperor was not deposed, nor did he receive any punishment whatsoever. This was largely due to America's fear of destabilization in Asia due to the rise of the CCP in China and the looming fall of the Kuomintang, thus they let Japan off the hook in return for its alliance as a democratic nation in East Asia. Because Japan was not formally punished for its atrocities in WWII, many in East Asia were not satisfied with the outcome, especially Korea and China. Though Japan has made many formal apologies about its conduct in WWII, even more than Germany, the lack of formal punishment does not sit well with the victims of the atrocities.

hi all. This can be an emotional topic, so just a reminder to potential respondents that this is a history sub, where we're looking for in-depth and informed responses from people with expertise in the subject matter. Source material citation is strongly encouraged. Discussion of events/conditions within the last 20-years are not permitted, nor are conjecture, opinion, and rumour. Thanks!

There have been some pretty good answers in this thread but they really tell only some part of the story-for instance, if the reason for Korean antagonism against China and Japan is purely due to mutual imperialism, then one would expect Taiwan, which also was a Japanese Imperial territory and claimed by the Chinese, to also be antagonistic to the Japanese and the Chinese. In fact, the Taiwanese probably have some of the warmest relations with both the Chinese and the Japanese in Asia.

The answer lies partly post-World War II. Because of very effective Japanese pseudo-assimilation policies, a large number of Koreans and Taiwanese either identified as Japanese or were supportive of Japanese rule. However, after the collapse of the Japanese Empire, there was an ensuing backlash in all territories-under war occupancy (like China) or civilian administration (like Korea and Taiwan). Nationalist Chinese punished Japanese collaborators (the "running dogs," or zougou), and on Taiwan, which was returned to China after the war, they also continued this anti-Japanese policy. Meanwhile, in Korea, a similar sentiment was going on, with especially strong resentment against the Chinilpa, Koreans friendly to Japan, who were often in positions of influence immediately after the war. However, the Cold War dramatically shifted the attitudes of both countries. Whereas on Taiwan, an exiled Nationalist government grew closer to the West-especially Japan and the US-in a front against mainland China (for instance, a Japanese officer helped organize the decisive defense of Quemoy against a Communist attack), in Korea, there was a significant amount of resentment that Korea had been torn asunder between communist rule supported by the Soviets and especially China, and the West, represented by the US. And blame was also placed on Japan, as it was argued that Japan's occupation of Korea had created a power vacuum that allowed these competing ideologies to swoop in. By blaming Japan-especially as new information about Japanese atrocities came to light-the political administration at the time was able to deflect blame from some of the rather oppressive and dictatorial policies that both the North and South implemented (until the gradual liberalization of South Korea in the 1960s and 1970s).

So while Japanese atrocities and oppression of Korea definitely continues to haunt full Korean-Japanese reconciliation, geopolitical realities play a role in either mitigating the fallout (as in the case of Taiwan and, in Europe, the Franco-German rapprochement post-WW2) or exacerbating it (as it does in Korea and Mainland China)

If you are in or near Seoul, I suggest visiting the Seodaemun Prison museum. It is obviously biased, being based on Korean scholarly work which is often at odds with Japanese figures and interpretation. But it illustrates well part of the colonial influence on Korean national memory, and why Koreans are reluctant to "let sleeping dogs lie."