WASHINGTON — When should a president say he's sorry?

President Obama's apology to his Afghan counterpart, Hamid Karzai, has resurrected the politically vexing issue of national contrition at a delicate moment in the war in Afghanistan and in the presidential campaign at home.

The president's "I'm sorry," for U.S. military involvement in the burning of copies of the Quran, has resonated in Afghanistan and on the campaign trail, where Republicans have been using it to support their claim that he is more interested in apologizing for U.S. mistakes than in defending U.S. power.

But Obama's decision to apologize sprang from a mix of principle and pragmatism, the hallmarks of presidential apologies over the years.

The mostly partisan outcry over Obama's apology shows the challenge he faces as a candidate for re-election and as a wartime commander in chief, roles whose motivations are sometimes at odds. In this case, his attempt to assuage angry Afghans with an apology he hoped would protect U.S. troops allowed some conservatives to question the strength of his leadership.

Speaking loudest from the Republican field, former House Speaker Newt Gingrich called the apology an "outrage," noting that on the day it was announced, two U.S. soldiers were killed in eastern Afghanistan during rioting over the incident. He said that if Karzai did not apologize for those deaths, "we should say goodbye and good luck."

For much of the past year, Republican candidates have mauled Obama's management of foreign policy. Former Massachusetts Gov. Mitt Romney has said Obama "went around the world and apologized for America." Romney called his campaign-style book "No Apology" to draw the contrast.

Much of the criticism stems from the more humble tone Obama has sought to bring to U.S. foreign policy after the swaggering approach of his predecessor, George W. Bush. He has banned the harsh techniques that the International Committee of the Red Cross called torture from U.S. interrogation policy and made clear that he believes living up to American values is an essential source of the nation's power. Repairing U.S. relations with the Islamic world has been a foreign-policy priority.

Approval rating

Polls show most of the country approves of his handling of foreign affairs, and administration spokesman Jay Carney called the criticism of the Karzai apology "fallacious and ridiculous narrative."

"There's the risk of opening yourself to political attack, but obviously for a president they have to make that calculation," said Robert Dallek, a presidential historian, who in defending the apology cited President James Monroe's declaration that "national honor is the national property of the highest value."

When and why a president has chosen to say "I'm sorry" have varied over the years, and Obama's apology to Karzai, a longtime ally in a decade-old war, is among only a few in recent decades that have been delivered in real time.

In 1988, President Reagan, held up by today's Republican field as the embodiment of U.S. self-assurance, apologized for the internment of Japanese Americans during World War II.

He signed legislation that eventually disbursed $1.6 billion in reparations to those affected by the policy.

Stephen Hess, presidential scholar at the Brookings Institution, said: "The trick is always to apologize for something the country did when you weren't president. What is remarkable is apologizing for something that has happened while you were president."

That same year, for instance, Reagan declined to apologize after missiles fired from the USS Vincennes downed an Iranian passenger jet, killing all 290 people on board.

He expressed regret for the loss of life but not for the event itself. He also did not apologize to Iran, a nemesis, showing that who is on the receiving end often determines whether an apology is issued.

Although no political party has a monopoly on the presidential apology, Republicans, more often than Democrats, have equated a lack of public regret with strength.

Reagan's vice president, George H.W. Bush, who was running for president in 1988, said at the time of the Vincennes disaster that he would "never apologize for the United States of America. Ever. I don't care what the facts are."

Bush was known as a moderate Republican, experienced in foreign policy. But he lived up to his pledge.

His son, however, strayed. George W. Bush said sorry several times as president.

In November 2002, he apologized, through the U.S. ambassador in Seoul, several months after a U.S. military vehicle hit and killed two South Korean girls.

According to news reports at the time, he also said he was sorry — privately — to then-Crown Prince Abdullah, of Saudi Arabia, for published articles saying Bush was unhappy with the kingdom's help on his declared war on terrorism.

Repeated apologies

The king of presidential contrition, though, was Bill Clinton, who apologized repeatedly over his two terms in office for national policy, past and present, and his behavior.

Clinton apologized for historic mistakes including slavery, the Tuskegee syphilis study and U.S. support for a Guatemalan government that carried out human-rights atrocities during decades of civil war.

He also apologized for ones that happened on his watch, such as the deaths at an Italian ski resort after a low-flying U.S. warplane severed a gondola cable, his lies about his dalliance with Monica Lewinsky, and his failure to act as the genocide in Rwanda unfolded.

Obama declined to apologize in November after NATO warplanes killed two dozen Pakistani soldiers near the border with Afghanistan, an incident a Pentagon investigation concluded was the result of mistrust and miscommunication.

U.S. military responsibility for the burning of the Qurans was more clear-cut than in the NATO airstrikes, and there were more urgent security factors at stake in Afghanistan. Administration officials say Obama's decision to apologize to Karzai was, in part, an attempt to soothe Afghan feelings and protect U.S. troops.

On Friday, Peter Lavoy, a senior Pentagon official, visited a mosque in Northern Virginia to apologize for the incident as violent protests spread in Afghanistan.

For the first time since word surfaced that U.S. soldiers had burned Qurans, protests reached nearly all parts of that country, with demonstrators chanting "Death to America" and "Death to the infidels." In Herat, in western Afghanistan, hundreds attempted to storm the U.S. consulate. Violence also was reported in the northern province of Baghlan, the eastern province of Nangarhar, in Khost province and in Kabul.

Afghan officials said at least 11 people were killed and scores wounded in Friday's protests. More than 20 people, including the two U.S. soldiers and one Afghan police officer, have been killed in the four days of violence.

Nonetheless, U.S. officials in Washington said they were cautiously optimistic that the tensions over the Quran burnings would soon ease. But they kept U.S. troops on heightened alert and reduced the number of patrols.

The criticism over Obama's apology to Karzai has been confined largely to Republicans running to replace him. Other conservatives have defended Obama's decision, given the importance of maintaining a working relationship with Karzai as U.S. forces begin winding down the war.

"It was an important demonstration of respect for the Afghan people and their religious faith," said Lisa Curtis, a senior research fellow at the conservative Heritage Foundation who specializes in South and Central Asia. "I think the United States dealt with this appropriately, but I also don't think any additional apologies are necessary."

Material from The Associated Press

and McClatchy Newspapers

is included in this report.