This week, Barack Obama became the first sitting president of the United States to visit the Japanese city of Hiroshima. A U.S. president visiting the site where the first nuclear bomb was dropped is significant enough, but during the lead-up to the visit, both American and Japanese officials were careful to make sure that no one expected Obama to issue a formal apology for the bombing. While there won't be an apology for the devastation the bombs caused in Hiroshima and Nagasaki, in recent decades the U.S. has taken steps to apologize for some significant actions it took part in over the centuries.

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Here are five instances where the U.S. government formally apologized for its actions:

Shielding a Nazi Officer Wanted for War Crimes

During Germany’s occupation of France in World War II, Klaus Barbie was one of the most infamous Gestapo officers. Nicknamed “the Butcher of Lyon,” Barbie was responsible for overseeing the murder and torture of French Jews and members of the French Resistance, as well as deporting thousands of Jews and noncombatants to concentration camps. When the war came to an end, Barbie slipped out of Germany and fled to Italy, then to Bolivia in 1951.

In a 1983 investigation spurred by charges from the French that the U.S. government had shielded Barbie after the war, officials with the U.S. Justice Department discovered that he had, in fact, been protected by several high-ranking members of the U.S. Army during the post-war occupation of Germany, Stuart Taylor, Jr. reported for the New York Times. The investigation found that the Army had been using Barbie as a paid informant during the last few years of the war and helped him escape to Bolivia in order to hide this fact from the public. To do so, the Army lied in response to inquiries about Barbie’s whereabouts, though the report found that they cut ties with the Nazi officer after relocating him to South America. In 1983, Barbie was finally extradited to France to face trial for war crimes. The U.S. issued a formal apology for hiding him later that year.

The Internment of Japanese Citizens During World War II

The decision by President Franklin Roosevelt to round up Japanese citizens and imprison them in camps came just 10 weeks after the bombing of Pearl Harbor, but the effects resonated through American culture for decades. During World War II, 120,000 Japanese-Americans and permanent residents were forced to abandon their homes and belongings in order to live under guard at several camps scattered throughout the U.S. due to racist fears that these citizens were loyal first and foremost to Japan. Not only were Americans deprived of their homes and belongings, but they were offered little during the decades after the end of the war to make up for their wrongful imprisonment.

Thanks to the activism of Japanese-Americans like Yuri Kochiyama (who was given a Google Doodle treatment recently in honor of her 95th birthday), in 1988, President Ronald Reagan signed the Civil Liberties Act, which offered every Japanese-American interned in the camps during the war a formal apology and $20,000 in compensation, Story Hinckley writes for the Christian Science Monitor.

The Overthrow of the Kingdom of Hawaii

In January 1893, a group of American-born businessmen and sugar magnates staged a coup against the Hawaiian Queen Lili’uokalani. Backed by American marines, the insurgents forced the Queen to abdicate and dissolve the Kingdom of Hawaii, setting the former island nation on the path to eventual statehood. While the coup’s backers quickly declared the country a new Republic, their true goal was to be annexed by the U.S. They got their wish in 1898, when Hawaii was formally annexed by the U.S. and administered as a territory until 1959.

One hundred years after the seizure,Congress issued a joint resolution formally apologizing to the people of Hawaii for the U.S. government’s role in the coup on November 23, 1993, as the New York Times records.

The Tuskegee Experiment

During World War II, scientists working for the U.S. Public Health Service started one of the most infamous and unethical medical studies undertaken during the 20th century: the Tuskegee Experiment. Starting in 1932, doctors working with the Tuskegee Institute in Alabama began conducting a long-term study on hundreds of black men to learn about the long-term progression of syphilis. The men were never informed that they were subjects of a study, and were never actually given the medical treatments that doctors told them they were receiving. In exchange for their unknowing participation, 399 black men living with syphilis were instead given meals, free medical exams, and free burial services after their deaths.

The study was originally meant to last six months, and it ended up running for 40 years—long after the discovery of penicillin and other medical treatments for the disease, according to the Centers for Disease Control. The experiment was uncovered by an investigation by the Associated Press in 1972, resulting in a $10 million settlement with the surviving subjects. President Bill Clinton formally apologized for the unethical experiments in 1997.

An Apology for Slavery and the Jim Crow laws

Few things compromised the core values of the U.S. Constitution and left as lasting a mark on American society as 246 years of institutionalized slavery and the subsequent discrimination of the Jim Crow laws that marked African-Americans as second-class citizens. As such, few people were more deserving of a formal apology than the millions of black Americans whose ancestors were forcibly brought to this country and had their freedoms stolen from them.

The formal apology for slavery and Jim Crow issued by the U.S. House of Representatives in 2008 was unprecedented, even after decades of lawmakers trying to push the government to finally apologize, NPR reported at the time. In introducing the resolution, Representative Steve Cohen (D-Tenn), noted that despite the government issuing an apology for interning Japanese citizens and later pressuring Japan to apologize for forcing Chinese women to work as sex slaves during World War II, the American government had never formally recognized and apologized for slavery. While the apology was primarily symbolic, by officially recognizing its role in perpetuating the horrors of slavery and Jim Crow, the American government took a step forward in addressing and atoning for one of its greatest wrongs.

Editor's Note: May 27, 2016: This post has been updated to reflect the correct date that a group of American-born businessmen and sugar magnates staged a coup against the Hawaiian Queen Lili’uokalani.