500
Technically, it's Japan. The westernmost point of Japan, on Yonaguni Island, is roughly 60 miles east of Taiwan.
North and South Korea are due west of Japan, across the Sea of Japan. North Korea is at the same latitude as the northern half of Honshu, the large central island of Japan. South Korea is at the same latitude as southern Honshu (and Tokyo), but farther north than either or Shikoku or Kyushu. The southwestern section of the Sea of Japan that borders South Korea is also called the Tsushima Basin, and the straits separating the countries at their closest point are called the Korea Strait.
China was interested in Korea because they saw them as an important trading partner and also a military outpost. Japan was interested for them to open 3 ports to trade and also similar to China's reason. They then made a hands-off agreement saying that they would not send their armies to Korea.
This point is on the northern island of Japan, about 18 miles north of Akita.
About 1200 miles from the closest point, about 1700 miles from the farthest point.
2063 miles at the closest point.
21 miles at the closest point
Due to it's close proximity and influence by China, Japan (who was at this point an emerging regional power) saw the Chinese-influenced Korea as a threat to Japan's Imperial expansion.
About 300 miles at the closest point.
About 140 miles at its closest point.
About 40 miles from the closest point.
At its closest point, Uranus is 1,607,000,000 miles from Earth.