(CNN)San Francisco's first and oldest sister city officially cut its ties with the Bay Area this week.
The split between Osaka, Japan, and San Francisco was initiated by Osaka Mayor Hirofumi Yoshimura.
The dispute between the two cities began in 2015 over the Comfort Women Memorial in San Francisco's Chinatown. "Comfort women" were women and girls forced into sexual slavery by the Japanese military during World War II.
The memorial features the statues of three women standing up and holding hands, one from China, one from Korea and one from the Philippines -- three countries where many "comfort women" were from. The statue is accompanied by educational tools that teach about the dark history of "comfort women."
"The San Francisco Comfort Woman Memorial is a symbol of the struggle faced by all women who have been, and are currently, forced to endure the horrors of enslavement and sex trafficking," San Francisco Mayor London Breed said in a statement. "These victims deserve our respect and this memorial reminds us all of events and lessons we must never forget."
Yoshimura announced the official split between the cities in a letter sent to the San Francisco mayor.
Although the letter addressed the "unforgivable act that violated the dignity and human rights of women," it claims Japan was not the only country guilty of "sex on the battlefield," citing other nations such as the United States, Great Britain and Germany.
Yoshimura claims that Japan's apology has been sincere enough, and pointed to a 2015 agreement with South Korea where Japan gave $8.7 million to a fund that helps WWII sexual slavery victims.
Julie Tang, a retired judge of the San Francisco Superior Court and the co-chair of the "Comfort Women" Justice Coalition, said the women were victims of human trafficking organized by the Japanese government during World War II.
"It is a Japan problem, because Japan has not owned up to it," said Tang. Japan "institutionalized" its sex trafficking during the war, whereas other countries did not, she added.
"This history is beyond description, and it has to be remembered. It cannot be shunted aside just because Japan wants it so," Tang said.
A spokesman from the San Francisco mayor's office called Yoshimura's decision "unfortunate" and said they will remain sister cities with Osaka "via people-people ties."
"One Mayor cannot unilaterally end a relationship that exists between the people of our two cities, especially one that has existed for over sixty years," the mayor said in a statement.
In the letter, Yoshimura wrote that if San Francisco ever decides to remove the memorial, Osaka will be open to continuing the sister city relationship. For now the statue isn't going anywhere, the San Francisco mayor's office told CNN.