Next time you visit the Houses of Parliament in Westminster, you will notice a difference: along with ever more obtrusive anti-terrorist defenses, you will see a life-size bronze statue of Nelson Mandela. The former South African president and Nobel Peace laureate has been in London this week for the unveiling of this statue. Prime Minister Brown has made the most of Mr. Mandela’s presence, hailing him as “the greatest and most courageous leader of our generation.”

To have a monument erected in such a place during one’s lifetime is unique. Churchill, the last prime minister of Britain to be similarly honored, had to wait until after his death. Mr. Mandela has played an important part in the history of his country, for which he has become an inspiration not only in his own continent but for the African diaspora throughout the world. As a foreign statesman he has, however, at most a peripheral role in British history — far less significant than, say, Franklin Roosevelt — and it is at least questionable whether or not he deserves Mr. Brown’s fulsome accolade.

Like Mayor Livingstone, who has ensured that London already has at least half a dozen streets named after Mr. Mandela, the prime minister is playing politics with such symbolism. By identifying himself with the only Left-wing hero whose reputation survived the Cold War intact, he sends a reassuring signal to the Labor Party, as well as reinforcing his multicultural credentials.

But Mr. Brown’s exaggerated claims for his hero prompt a question that has never been more urgent: what makes a leader great in today’s world?

As it happens, Mr. Mandela belonged to a generation that was blessed with courageous leadership. John O’Sullivan has recently highlighted the trio of Ronald Reagan, Pope John Paul II, and Margaret Thatcher, who among them all turned the tide of history during the 1980s. It seems to me beyond dispute that all three of these leaders showed extraordinary courage in confronting the enemies of civilization, and it is no accident that all three survived assassination attempts.

Some historians would add the name of Mikhail Gorbachev to that list, but I would dissent from that. Compared to his predecessors or his counterparts in China, the last Soviet leader did show restraint in dealing with “people power,” but all his efforts were directed toward preserving the “evil empire.” The real courage and greatness was shown by the leaders of the dissidents: men like Andrei Sakharov and Natan Sharansky, Lech Walesa and Vaclav Havel.

What of our own time? The jury is still out of course on leaders of the recent past or present. But it is already possible to identify a few figures who demonstrated courage and even greatness at decisive moments.

The late Boris Yeltsin, when he rallied the forces of democracy to crush the attempted coup from the top of a tank in 1991, was one such. Rudy Giuliani showed similar qualities during the hours and days after September 11, which is why I fully expect him to defeat either Hillary Clinton or Barack Obama if he makes it to become the Republican presidential candidate next year.

Great leadership is not restricted to great powers. John Howard, the prime minister of Australia, has proved himself the bravest leader his country has so far produced by his resolute stand against terrorism. There are great dissidents alive today, too, such as Aung San Su Chi and Ayaan Hirsi Ali, who have spoken up for the victims of Burmese and Islamist tyranny respectively.

But the two leaders who excite most controversy are surely Tony Blair and President Bush. I am skeptical of what Mr. Blair can usefully contribute in his role as Middle East “peace envoy,” but in his 10 years as prime minister he showed courage amounting to greatness on three occasions when it mattered: in confronting the Milosevic regime during the Kosovo crisis, in siding with America against the Islamist jihad, and in refusing to abandon Iraq or Afghanistan.

Mr. Bush is even more difficult to assess, for the final 16 months may turn out to be the most important of his presidency. We can, however, already discern its turning point: facing the challenge of the most devastating attack on America since Pearl Harbor, he decided to respond to terrorism that backs an “axis of evil” with a democratic mission to overthrow the Taliban and Saddam regimes by force.

Even if this president had never done anything else, his break with global consensus was a rare moment of visionary greatness. If he had been struck down by an assassin’s bullet in 2003, Mr. Bush would, like John F. Kennedy, be among the most popular presidents in history, rather than one of the most unpopular. With the insurgency in Iraq seemingly unstoppable, with both houses of Congress lost, and with even his most loyal supporters falling away, this increasingly lonely but still resolute president has stayed the course.

In his speech on Tuesday to the American Legion at Reno, Nev., Mr. Bush warned that he had authorized American commanders to halt “Iran’s murderous activities,” adding, “We will confront this danger before it is too late.”

This sounds like a president who is preparing the world for military action against the Iranian nuclear threat. If so, Mr. Bush will need to show once again the qualities that steadied America six years ago next month. Only a great president will save the world from “the shadow of a nuclear holocaust.” What may well be Mr. Bush’s finest hour has come, as it does for most leaders, in adversity. I am confident that he is equal to the task.