Even a congressional resolution of apology doesn't carry the same emotional weight.
President Ronald Reagan was initially reluctant to apologize to Japanese-Americans who were imprisoned in camps during World War II. He did so after Congress issued its apology and provided for reparations.
Bush apologized for abuse of Iraqi prisoners at Abu Ghraib after the photographic evidence was seen around the world. He called it "a stain on our country's honor and our country's reputation"
After years of pressure by Native Americans, in 2009 Congress passed, and Obama signed, a resolution apologizing "on behalf of the people of the United States to all native peoples for many instances of violence, maltreatment and neglect."
At other times presidents have admitted things have gone wrong, giving the impression of an apology while stopping just short: Bush on his administration's flawed response to Hurricane Katrina, Richard Nixon regretting the Watergate break-in, Reagan on the arms-for-hostages scandal.
Instead of a simple "I'm sorry," Reagan offered: "Mistakes were made."
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