South Korea, despite the threat of North Korean looming over Seoul, has built a formidable navy:
With the world’s attention focused on a potential confrontation between China and Japan in the East China Sea, a third player has built what may be the most powerful ship-for-ship fleet in Northeast Asia. Over the past fifteen years, the Republic of Korea Navy (ROKN) has expanded dramatically, acquiring a substantial fleet of modern, powerful warships. While the ROKN continues to prepare for the contingency of conflict with North Korea, it has become a force capable of significant foreign deployment. If Seoul maintains its commitment to the Navy, the ROKN could become one of the world’s premier middle power navies.
I cited Strategypage 5 years ago in noting South Korea's naval building program that is making them a formidable force on China's door step.
South Koreans may resent Japan for the pre-1945 colonial era, but China's little pet psycho regime is currently threatening nuclear war with South Korea and has repeatedly threatened to turn Seoul into a "sea of fire."
China will always have to worry that South Korea will join with Japan in resisting Chinese aggression.
Oh, and if I ran Vietnam, the Philippines, Taiwan, and every other country with claims in the South China Sea, I'd start fortifying my claims the minute the shooting starts between China and Japan.
The thought of that will keep much of the South Sea Fleet occupied, I imagine.