Hello, very sorry to respond to this so long after you’ve asked it. I meant to respond earlier but the holidays, and then catching up with work afterward, dragged the research and writing process out. It seems like within this post there are in fact several questions so I will try and address them to the best of my ability.

Firstly, as regards the post you linked, I am actually rather surprised by that opinion being shared. While I don’t consider myself an expert on online right-wing Japanese rhetoric, this is the first time I have seen claims that “Korea is not a colony; it is Japan.” I think a far more typical stance taken by the Japanese far-right (and even those unknowingly parroting these points) is that Koreans are ungrateful for the modern amenities brought to the peninsula under Imperial Japanese rule. Things such as waterways, hygiene, railroads, modern education systems, and the like are typically brought up as things Korean take for granted now but were introduced under Japanese administration. That is to say, a typical argument of those who hold their imperial past in high regard and resist push-back from those who raise the social (and in many cases material as well) consequences of empire. Having said this, however, I wonder if the statement refers more towards the land of Korea, rather than people. Japanese arguments tend toward exceptionalism and draw on how Japanese are distinct from other ethnicities. I think, however, that I am not qualified to say much more than this on the subject, and to do so would detract from the rest of what I want to write.

The next question is if Japan’s annexation of Korea was based on views of Korea as an integral part of the Japanese nation, despite what appear to be significant cultural differences. In some ways, as I will show, yes. The rhetoric Japan applied towards its colonies, subjects, and the Japanese people changed as the Empire and its goals changed. As for the cultural differences, I’d like to refer to a line by Korean historian Bruce Cummings who points out that Japan and Korea, both as neighbors and as colonizer and colonized, are more similar to “France and Germany or England and Ireland than… Belgium and Zaire or Portugal and Mozambique.” While it’s true there were significant cultural and linguistic differences between the two, it should also be remembered they existed in the same Sino-centric cultural sphere together for over a millennium, and the literati of both could communicate via Chinese, the lingua franca of the region and time.

As for religion, I think two things should be said, firstly, caution should be used when using such culturally loaded words like that when talking about a space-time so removed from what a Western understanding of the word ‘religion’ might entail. Secondly, while indigenous rites and ceremonies may have differed, there were many shared ones, especially in the form of Confucianism, which I hesitate to call a religion, but which gave a shared set of customs that helped make sense of the aforementioned Sino-sphere.

Finally, for what reason did Japan have an interest in Korea? As I mentioned above, the goals of the Empire changed over time and so too did the way that it thought about its territories. Here I will try to demonstrate both the changes in political and well as racial thought applied by the Japanese Empire onto Korea.

Before I start, to briefly comment on your question about a recruiting pool, Koreans weren’t allowed into the Japanese military until 1938, first as volunteers, and later drafted in 1944. So in short, it’s a no to that possibility. Just thought I would mention that before going ahead with the rest of the post.

Prior to annexation in 1910, of course, Korea had been a Japanese colony since the end of the Russo-Japanese War in 1905. To go further back, however, and observe how Japan’s interest in Korea changed over time, we should start with the Meiji Restoration in 1868. During the same year that the Meiji Emperor replaced the old Tokugawa Shogunate, envies from the new Meiji government to inform Korea of this transfer of power were rejected by the latter. Among diplomatic exchanges between China, its tributary Korea, and Japan, the use of the character for ‘emperor’ 皇 was reserved for the Chinese Emperor. Korea rejected Japan’s use of this term for their new head of state on the grounds that it broke with centuries of diplomatic tradition. Further, accepting Japan as an empire would also be an implicit acceptance by Korea of a lower diplomatic status. Until then, leaders of Korea and Japan were both referred to as ‘kings’ of their respective lands when dealing with China and in understanding each other. Korea, which, saw itself as the cultural successor of Ming China and superior to Japan, could not accept Meiji Japan’s requests for recognition. Later attempts to establish ties between Korea and the new Japanese government were again rejected by the former due to further perceived breaks with tradition by the latter. The idea that Japan had lost their way in its pursuit to adopt Western customs also played a role. The result of these diplomatic failures was Korea closing its ports to Japanese ships and commerce and its continued non-recognition of the new Japanese government.

This slight against the Meiji government became a topic of debate amongst Japanese leaders in the 1870s that stopped just short of seeking a pretext to invade the Korean peninsula. Instead of going so far, Japan instead floated a gunboat, the Un’yō, into forbidden Korean waters, drawing Korean cannon fire. Japan used this as a pretext to land a dispatch of troops on Korean soil and demand a treaty be signed opening Korea to Japan. As pertains to your question, these episodes reflect an attempt by the Meiji leaders to show how well they had adopted the 19th century Western-centric liberal international order. This order claimed that nations (including empires) were the ultimate unit of diplomacy and that there was equality between said nations, big and small, weak and powerful. What is being witnessed in these diplomatic tiffs between Japan and Korea is a conflict between this foreign order and the Sino-centric one that had prevailed in East Asia for centuries. As a Chinese tributary, Korea was able to justify its aversion to establishing diplomatic ties by deferring its authority to China. Japan, however, was eager to settle nation-to-nation level agreements with Korea that would, nominally, allow them to negotiate on equal terms, without having to go through China.