In the early 20th century, some Koreans were close to the Chinese, some to the Russians and some to the Japanese. After the Japanese empire annexed the Korean Peninsula in 1910, Koreans faced a choice between almost hopeless resistance and collaboration. Much of the Korean elite chose the latter, not always for disreputable reasons. Cooperating with the Japanese was one way to modernize the country. Universities and newspapers were founded, railways built, industries established and so on.

But Koreans remained second-class subjects of the Japanese emperor and in the latter stages of colonialism were made to relinquish their culture and language and to conform to the Japanese. The humiliation of being forced into subservience to a nation often seen as a rival and never a master ran deep, and it still poisons relations between the two countries.

The same tangled history of submission and collaboration continues to have a toxic effect on domestic politics in South Korea. Park Chung-hee, the authoritarian leader who signed the 1965 agreement to let bygones be bygones, had been an arch-collaborator. He had served as an officer in the Japanese Imperial Army; right-wing Japanese politicians considered him a friend. One of them was Nobusuke Kishi, who became prime minister after having spent some time in jail as an indicted war criminal. (He was never tried.) Mr. Abe, one of the most nationalistic politicians since World War II, is his grandson.

Koreans on the left have never forgiven the conservative South Korean elite for its record of collaboration. They pride themselves on resistance against both the Japanese and right-wing authoritarians like Mr. Park. Most former collaborators and their offspring were members of conservative parties. The disgraced former president Park Geun-hye is Mr. Park’s daughter. She was driven from power and into prison because of corruption scandals, but the animus against her from the left also had much to do with her family background. Family is very important in Korean, as well as Japanese, politics.

Moon Jae-in, the current president of South Korea, is a man of the left, which is why it makes sense for him to break with the 1965 agreement with Japan. The reason is not simply a legacy of anti-Japanese resentment, made worse perhaps by the fact that Japan is now led by the grandson of a leader accused of war crimes, including the use of Chinese and Korean slave labor. More important is the bitterness against the political class in Korea that is still tainted by those crimes.

Mr. Abe, for his part, has his own political fish to fry. His grandfather tried to revise the postwar Constitution, written by the Americans in 1946, which bans the Japanese from using military force. Most Japanese then, happy with the country’s official pacifism, opposed Mr. Kishi’s attempt. Mr. Abe would dearly like to fulfill his grandfather’s wishes. A majority, if smaller now, of Japanese would probably still oppose such a move.

Lest one assume that Mr. Abe’s goal is only inspired by chauvinism, it’s worth considering that one of the first people to question the wisdom of Japanese pacifism was Richard Nixon in 1953, when he was vice president of the United States. Mr. Nixon believed that a revision to Japan’s pacifist Constitution would serve to turn the country into a more formidable ally against communism.