In a TV interview this week, John McCain offered President Obama some sound, if difficult, advice.

“Mr. President,” the Arizona senator said, “don’t be ashamed of re-evaluating your view of the role of the United States in the world.”

No one likes to admit having been wrong on a fundamental issue. For an American president, few things can be more difficult. When you are invested with tremendous power and prestige because you persuaded tens of millions of citizens to raise you to the highest office in the land, acknowledging that you blundered doesn’t come easy. All the more so when the blunder goes to the very core of your strategy for leadership.

Obama’s foreign policy is in a shambles. Nearly six years into a presidency whose approach to the world has been grounded in American retrenchment, “leading from behind,” deference to multinational organizations, and rejection of military solutions, the world has become a much more dangerous place. Exhibit A, of course, is Iraq, where Obama was not only adamant that all US troops must be withdrawn, but boasted – over and over and over – that he had kept his promise.

It is clear now that America’s disengagement from Iraq, coupled with Obama’s unwillingness to aid moderates in the Syrian civil war, created a vacuum that the vicious jihadists of ISIS readily filled. Their self-proclaimed caliphate now rules an estimated 35,000 square miles in Iraq and northern Syria. This month Obama reluctantly ordered targeted airstrikes near Irbil, and the Pentagon is considering potential bombing targets inside Syria. But the president still cannot bring himself to concede what more and more Americans grasp: The US retreat from global leadership was profoundly unwise.

Yet acknowledging error would be a mark of character. Other presidents have done it.

George W. Bush initially supported the view, advanced by Defense Secretary Donald Rumsfeld, that the US troop presence in Iraq would inflame the violent post-Saddam insurgency, and that the only strategy to reduce the bloodshed was to shrink the American military footprint. But in 2006, Bush reversed course. “It is clear that we need to change our strategy in Iraq,” he told the nation in a televised address, announcing the deployment of 20,000 additional troops. The surge was deeply unpopular – Bush calls it “the toughest decision” of his presidency. But it defeated the insurgency and won the war.

When Yugoslavia erupted in the 1990s and Bosnians were being brutally attacked by the Serbs, Bill Clinton offered little more than lip service to the victims. Not only was he was unwilling to act directly to stop the Serbs’ genocidal attacks, he wouldn’t even end the arms embargo that was leaving Bosnians defenseless. An increase in military action, the administration said, would bring peace “not an inch closer.”

But that attitude changed after the Serbian massacre of more than 7,000 Bosnian men and boys in the supposed “safe haven” of Srebrenica, and, later, a deadly attack on a marketplace in Sarajevo. After long resisting a military intervention, Clinton reversed course. The United States led a NATO bombing campaign that brought peace not just an inch closer, but ended the Bosnian War. Today, for all his flaws, Clinton is widely esteemed a hero to Bosnians.

Perhaps no modern president has been as forthright as Jimmy Carter in admitting that his approach to foreign policy was egregiously misguided.

Carter had come to office willing to believe the best of the Soviet Union and lecturing Americans on how they should get over their “inordinate fear of communism.” The Soviet invasion of Afghanistan in 1979 woke him up. Moscow’s aggression “has made a more dramatic change in my opinion of what the Soviets’ ultimate goals are,” Carter confessed, than anything he had previously observed. Soon after, he announced the Carter Doctrine, declaring that the United States would use military force if necessary to defend its national interests in the Persian Gulf. He also ordered a military buildup, setting the stage for Ronald Reagan’s further increases.

During the Civil War, Abraham Lincoln had no faith in Gen. Ulysses Grant’s strategy for capturing the stronghold of Vicksburg, Miss. When Vicksburg fell, Lincoln wrote Grant a letter owning up to his misjudgment: “I now wish to make a personal acknowledgment,” it ended, “that you were right and I was wrong.”

The 44th president, who cites the 16th as a role model, could do with some of that candor. Obama’s foreign policy didn’t lead where he expected it to, and there is no shame in admitting it.

(Jeff Jacoby is a columnist for The Boston Globe).