In regard to the letter submitted by Setsuko Tokita, although it is true that there was a long history of African slavery in the United States, the U.S. government finally put an end to slavery once and for all during the bloodiest conflict in American history, the Civil War. However, the institution of slavery even during this period in world history (the 19th century) was almost universal and could be found in various countries and cultures across the globe, including, it may be argued Japan, where peasant farmers lived as lowly serfs serving a daimyo and allowed few freedoms. As late as the 1930s, bankrupt farmers in Tohoku were selling their very own daughters into sexual servitude to save their farms from being foreclosed.
Tokita must understand that America fought a bloody civil war (1861-65) to end slavery and preserve the U.S. Japan, by contrast, set out in the 1930s to establish an Asian empire to be built in large part by slave laborers, including the use of sex slaves. This was all very much a part of the Imperial Army's plans for Asian domination and very likely inspired Nazi Germany to use millions of concentration-camp slave laborers in a like manner.
I might conclude by saying that America has never denied its role in the world's history of slavery. President Bill Clinton actually offered an apology to the nations of Africa when he visited there. Japan, on the other hand, has attempted since the end of World War II to keep sexual slavery a secret or simply deny that such atrocities ever took place.
And America has never tried to hide behind such ingenuous euphemisms as "comfort women" or "comfort stations" to conceal the very ugly truth about the African slave trade. In fact, the 1970s American television program "Roots" explored the brutal history of slavery. And the American educational system offers thousands of books and documents on the subject of antebellum slavery. Prime Minister Shinzo Abe, on the other hand, allegedly attempted to prevent NHK from broadcasting even one documentary about Japan's sex slavery.