Indeed Kagan, at 50, is in his prime. A monthly columnist for The Washington Post and senior associate at the Carnegie Endowment think tank, he was one of the chief foreign policy advisers to Republican presidential candidate John McCain.

He is a typical neo-conservative in many respects, not least in his moral clarity and conviction that America should be prepared unilaterally to declare war to uphold its values. But the label is meaningless to Kagan, who argues that there is nothing neo, or new, about his philosophy. He considers himself part of a long tradition of foreign policymakers who stress the importance of US global leadership.

In his 2006 book Dangerous Nation - the first of a projected two-volume history of US foreign relations - Kagan attempted 'to disprove the idea that America is traditionally an isolationist nation that only occasionally heads off into the world'. Some reviewers charged him with revising history to legitimise the neo-con vision of an imperialist America.

For all his rhetoric, he is surprisingly congenial. Large but baby-faced, with a wry smile, he introduces himself as 'Bob'. He recently returned from three years in Brussels, where his wife Victoria Nuland served as US ambassador to Nato.

Gore Vidal once said of Kagan that he is 'in the grip of a most unseemly megalomania, speaking for no one but political hustlers within the Washington beltway'. But his analysis of geopolitical trends regularly draws plaudits from his political adversaries. Henry Kissinger, whose realist outlook often opposes the values-based vision of neo-cons, called Kagan's 2003 book Of Paradise and Power a 'seminal ... discussion of European-American relations'.

As the world's lone superpower, the US naturally favours an international order where might prevails, Kagan argues in that book. By contrast, European countries - militarily weaker, more geographically exposed to the risk of war, and shadowed by the second world war - push for diplomatic solutions to military action, seeking regulation through international co-operation rather than the anarchy of nations.

Kagan has ignited fresh debate with his latest book, The Return of History and the End of Dreams, in which he proposes a global league of democracies. For Kagan, the resurgence of China and Russia as great-power autocracies calls for a forum where the world's 100-odd democracies can meet to advance their shared values.

The title alludes to Francis Fukuyama's hypothesis that history in the form of ideological struggle ended with the fall of the iron curtain and was replaced by inexorable market capitalist democracy. 'When we started the post-cold war period, we thought that there wasn't a challenge to democracy - that it was just an issue of fostering economic development,' Kagan says. 'But we have two powerful autocracies ... not undergoing this anticipated evolution based on economic growth. Democracies need to begin acting together in a more concerted fashion, whether it's dealing with problems like Zimbabwe and Burma [Myanmar], or showing solidarity against the resurgent ambitions of Russia.'

Few will agree with Kagan that he belongs 'very much in a bi-partisan mainstream', but it is true neo-cons cannot be neatly identified along party lines. In the 2000 presidential election, Kagan voted for Al Gore against George W. Bush's pledge to scale back US international commitments. 'I spent much of the 90s fighting against the Republican Party, which was opposed to intervention. It's only later that people have revised this history and created this fiction of a neo-conservative movement distinct from liberal interventionism, which has been the mainstream policy since the cold war.'

After September 11, 2001, Bush reversed his foreign policy and Kagan, who had long pressured the Clinton White House to force regime change in Iraq, was in demand.

Kagan was a teenager during the Jimmy Carter years, 'such a down period for America', he says. 'We heard all about the limits of American power and how the United States was in decline and fading.' When Ronald Reagan came to office in 1980, Kagan admired his 'refusal to accept that the Soviet Union was in a state of inevitable ascension and that there was no hope for the democratic world'.

He worked in the State Department during the mid-80s, while watching the falls of Augusto Pinochet and Ferdinand Marcos and cracks appear in the Soviet edifice. 'That was the period when we shifted from a policy of blindly supporting dictatorships to supporting more moderate and centrist democratic forces in Latin America and Asia, with some quite astonishing successes,' Kagan says.

'It's one of the reasons I don't share the general scepticism that there isn't anything we can do to support democracies in countries where there isn't democracy at this moment.'

Did it concern him that McCain's running mate, former Alaska governor Sarah Palin, seemed to know little about global affairs? 'She had at least as much foreign policy experience as some of McCain's other potential vice-presidential choices,' Kagan says, 'but because they were men, no one was raising any questions.'

So ignorance about foreign policy is the norm among McCain's inner circle? Kagan smiles. 'This whole idea that only a certain elite foreign-policy community can be trusted with American foreign policy is false. I trust the average American to make better decisions about many of these issues.'

Asked about the challenges involved in reviving US legitimacy on the world stage, Kagan says: 'Most nations act according to their interests, and their interests are not necessarily affected by who's president of the United States. The behaviour of governments in the international system is not fundamentally anti-American.'

Asian countries increasingly look to the US for protection against China, he says, adding that America enjoys much closer relations with Europe than two years ago. The attempt by France and Germany to counterbalance US power by embracing Russia failed, Kagan contends, under pressure from the new eastern and central European EU member states, whose anxieties about the Kremlin have increased since its military actions in Georgia.

According to Kagan, there are two democratic clubs in existence - Nato and the EU - but they fail to reflect the new global reality of democracies dispersed throughout the world, whereas his proposed democratic concert will incorporate countries in Latin America, Africa and Asia.

The UN Security Council may be 'hopelessly paralysed', as Kagan puts it, but is it not safer to have major world powers united under a single organisation, where democracies are forced to negotiate with illiberal regimes, than split into two competing camps?

Writer's notes

Name: Robert Kagan

Age: 50

Born: Athens, Greece

Lives: Washington, DC

Family: married to Victoria Nuland, formerly the US ambassador to Nato; children Elena, 11, and David, nine

Genres: political commentary; history

Other jobs: columnist for The Washington Post; senior associate at the Carnegie Endowment for International Peace; formerly a State Department official

Latest book: The Return of History and the End of Dreams (Knopf; Atlantic Books)

Other books: A Twilight Struggle: American Power and Nicaragua, 1977-1990 (1996); editor with William Kristol of Present Dangers: Crisis and Opportunity in American Foreign and Defence Policy (2000); Paradise and Power: America and Europe in the New World Order (2003)

Next project: the second volume of his history of US foreign relations

What the papers say: 'A brief and wonderfully argued volume on how the world has a nasty habit of spinning off in its own directions ... Kagan barely mentions Bush - his wife's boss, after all - as he brutally dissects the argument that freedom is still on an unstoppable march.'

The New York Times on The Return of History and the End of Dreams