The bone of contention in the Korean peninsula is the impact of the war that happened more than 50 years ago. The painful memory of an invasion, the humiliation and the forced sepearation. The past is still fresh in the minds of the countries involved.
Korea before it divided was once a colony of Japan. This was from 1910 to 1945. Earlier in 1905, the then-Korean Empire was declared a protectorate of Japan. And Japan was not gentle about the invasion as it looted, killed and maimed.
In 1919, a Korean movement for independence was crushed. The leaders flee to China where they built an army and attacked again with the help of the Chinese Nationalist Government. At this time Japan was the great bully in Asia and by then was a world power as the United States is now. She was also at war with China who she had earlier invaded.
It is not surprising the new romance Japan enjoys with the US who she shares a similar foreign policy with. Both were or are international bullies. As both contributed to the breakup of Korea. As for China, she still holds dear her old partnership with Korea and may never give up on his friend anytime soon