The United States recently proposed that South Korea and Japan link their ballistic missile detection systems with the American one and all information collected, especially in real time, about missile launches in North Korea would be instantly available to all three nations. All three countries have radars that can spot missile launches in North Korea, with some of the American ones being satellite based. All three have Aegis equipped warships carrying anti-missile missiles. On short notice a dozen of these ships can be assembled off North Korea. In addition South Korea and Japan are each increasing their land-based anti-missile capabilities.
While the American proposal appeared to be a good one, Japan and South Korea declined to participate. What is really going on here is continued Korean anger over brutal Japanese colonial occupation from 1910-45, and centuries of Japanese aggression towards Korea. Then there are the conflicts over names of islands and the waters separating the two countries as well as the ownership of some
These problems have existed for a long time. In 2009 South Korean politicians were in an uproar because the electronic maps supplied with the American F-15K fighter-bombers South Korea purchased included the wrong names for some geographical features. In particular, South Koreans were upset that the U.S. maps called the Dokdo islands by its old Japanese name, and what the Koreans call the "East Sea" was labeled as the "Sea of Japan."
This was serious stuff. In 2008 South Korea recalled its ambassador from Japan, because of escalating confrontations over the disputed ownership of some islands. South Korea has long been willing to sacrifice good relations with Japan over the issue of who owns the uninhabited Dokdo (Takeshima to the Japanese) islands in the Sea of Japan (East Sea in Korean).
To Westerners this dispute may appear somewhat absurd. In the West these islands are called the Liancourt Rocks, which more accurately describes what they are. Increasingly both countries have been sending more air naval reconnaissance missions to the islands and the mass media in both countries have been jumping all over the tension.
The Liancourt Rocks are midway between South Korea and Japan, although an inhabited South Korean island is closer than the nearest inhabited Japanese island. Since the 1960s a few South Korean civilians and a growing number (now about 40) of South Korean police, light house keepers and other government officials have been stationed on the islands. Those on the island must be supplied by boat, although you can catch rainwater in cisterns and the fishing around the Liancourt Rocks is excellent. That’s one reason both nations claim ownership. There’s always the potential for oil or other natural resources be discovered in the seabed around the rocks.
National pride prevents either nation from backing off on its claims and this dispute makes it difficult to participate in military cooperation, even when it means improving defenses against North Korean or Chinese attack. Both nations are willing to cooperate with the United States, just as long as such arrangements do not include their neighbor across the Sea of Japan/East Sea.