I'm currently reading The Clash: U.S.-Japanese Relations Throughout History by Walter LaFeber. I've just gotten to the part which briefly goes over the Sino-Japanese war of 1894-95. That combined with some history pieces at the Marmot's Hole (on some Koreans who sold out to the Japanese) and Budaechigae (on Queen Min) got me hyped about doing my own digging.
First, here is a very brief history of the war:
1894–95, conflict between China and Japan for control of Korea in the late 19th cent. The Li-lto Convention of 1885 provided for mutual troop withdrawals and advance notification of any new troop movements into Korea. Accordingly, when a Korean revolt erupted in 1894, both countries sent troops. However, after the insurrection had been suppressed, Japan refused to withdraw its troops and induced the Korean court to abrogate its agreement with China. The fighting that ensued between Chinese and Japanese forces ended with an easy victory for the more modern Japanese army.
I just want to go over that first sentence again: 1894–95, conflict between China and Japan for control of Korea in the late 19th cent. Japan's colonization of Korea is relatively well known but China's own imperialistic relationship with the Chosun Dynasty is much less well-known and generally swept under the rug by Chinese nationalists who insist that there was no such thing as Chinese imperialism.
Another sign of the relationship between China and Korea is indicated in Michael Breen's book, The Koreans. Breen states:
In 1873, the Tanwon-gun [regent] was retired and King Kojong began to face the inevitable question of how to deal with the outside world. Japan pressured the throne into signing its first modern treaty, one which cleverly identified Korea and Japan as independent states, thereby undermining Chinese claims over Korea. (page 99)
Of course, such a declaration would hardly be necessary if China did not excercise suzerainty over Korea. The treaty between the Japan and Korea was hardly equal, however and Japan eventually colonized Korea.
Korean scholars also acknowledge Chinese influence over Korea. Check out these two blurbs from an article by Kim Young-Sik, at the Association for Asia Research web page. The first demonstrates Korea's protectorate status with China:
The Chosun court archives (kojong silrok) show that in 1855 and also in 1865, a number of shipwrecked American sailors were picked up on the Korean shores. They were fed and treated well by the Korean populace, and then sent to China for repatriation to the United States. In those years, Korea sealed itself in ("Hermit Kingdom") and let China handle Korea's foreign affairs.
However, by the time Korea and the United States signed a friendship treaty, Chinese influence over Korea had lessened considerably:
It should be noted that Commodore Shufeldt rejected the Chinese request to incorporate Chinese suzerainty over Korea in the treaty, and he made it clear that the United States recognized Korea as an independent nation.
So it seems pretty clear to me that China exercised suzerainty over Korea until the later was opened up (and later colonized) by Japan. Interestingly enough, that is a similar relationship that China had with Tibet until 1949 and we can all see the relationship between those two nations today.
(UPDATE: Go on to the comments section. It has some good stuff.)