A WEEK of absurdity around a confused racial con artist, and a massacre in a black church brings us to this: Today is the 150th anniversary of Juneteenth, when the last of the American slaves were told they were free. Now, to put it to good use, at a time when a post-racial era seems very much out of reach.

The first black man to live in the White House, long hesitant about doing anything bold on the color divide, could make one of the most simple and dramatic moves of his presidency: apologize for the land of the free being, at one time, the largest slaveholding nation on earth.

The Confederate flag that still flies on the grounds of the Statehouse in South Carolina, cradle of the Civil War, is a reminder that the hatred behind the proclaimed right to own another human being has never left our shores.

An apology would not kill that hatred, but it would ripple, positively, in ways that may be felt for years.

As the son of a Kenyan father and a white mother who died more than a century after slavery ended, Barack Obama has little ancestral baggage on this issue. Yet no man could make a stronger statement about America’s original sin than the first African-American president.

Conservatives would caw — they always do — and say, get over it, don’t play the race card. Liberals would complain that a simple apology did not go far enough, unless it entailed reparations for the descendants of slaves. But words of contrition — a formal acknowledgment of a grievous wrong by a great nation — have a power all their own.

The British, the Vatican, the Germans and the South Africans have all issued formal apologies for their official cruelties, and each case has had a cleansing, even liberating effect. The United States Congress apologized to African-Americans for slavery in 2009, though it came with a caveat that the mea culpa could not be used as legal rationale for reparations.

And President Bill Clinton, while in Africa in 1998, apologized for the slave trade, but not for a government that institutionalized white supremacy during its first four score and change.

For this year’s Juneteenth — commemorating the day in 1865, more than two years after the Emancipation Proclamation, when a Union general landed in Galveston, Tex., and told the last of the dead-enders in Texas that “all slaves are free” — President Obama could close a loop in a terrible history. He could also elevate the current discussion on race, which swirled earlier this week around the serial liar Rachel Dolezal, and the race-baiting billionaire vanity blimp of Donald Trump.

The slaughter of worshipers in a church with long ties to fighting slavery and Jim Crow “raises questions about a dark part of our history,” President Obama said Thursday. Questions about why South Carolina can still fly the flag of a traitor nation, a flag apparently embraced by the shooter. Questions rooted in a history that calls for a formal apology.

From the time the first Africans arrived as slaves in Jamestown in 1619 through the codification of blacks as three-fifths of a person in the Constitution and up to the eve of the Civil War, when four million people were held in bondage, slavery has been the most incongruent element of a people proclaiming to be enlightened.

Lincoln said he hated “the monstrous injustice of slavery,” in part because it allowed “enemies of free institutions, with plausibility, to taunt us as hypocrites.” Countries, religions and corporations sometimes do awful things in their names. It doesn’t diminish them to note their failures, their injustices, their crimes against humanity. It elevates them.

When Prime Minister Tony Blair of Britain apologized to the Irish in 1997 for England’s role in a famine that killed more than a million people, it opened the door to reconciliation, and a burst of new scholarship and awareness about a genocidal episode long mired in shame.

The British government also tried to make good for prosecuting a World War II hero and Nazi code breaker, Alan Turing, for the crime of being gay.

It took the Vatican 350 years to apologize for the persecution of the Italian astronomer Galileo. But now the church speaks with authority, backed by science, on climate change — leaving Republicans in the United States in the dungeon of ignorance.

Pope John Paul II apologized to Jews for the Vatican’s inaction on the Holocaust and to Muslims killed by crusaders. Last year, Pope Francis reached out to victims of clerical sex abuse and said he was sorry on behalf of the church he leads.

President Ronald Reagan signed legislation that provided payments and apologies for the internment, during World War II, of more than 100,000 Japanese-Americans — most of them United States citizens.

It’s harder to be contrite than to conquer. Obama had nothing to do with slavery. Most Americans, descendants of immigrants shunned in their homelands, have very little connection to the slaveholders of the American South. So why apologize? Because we own this past. As such, we have to condemn it.