Margaret MacMillan says historians are "pains in the neck. 'That wasn't really true', 'It wasn't quite like that' - we can be intensely irritating. And when people ask what was true, we say we don't know!"

MacMillan has been warden of St Antony's College in Oxford since 2007, but she is also an acclaimed historian, author of Women of the Raj (1988), Peacemakers: The Paris Conference of 1919 and Its Attempt to End War (2001) and Nixon in China (2006). She has now produced a lively and wide-ranging study, The Uses and Abuses of History.

History, she suggests, can be dangerous, particularly for dishonest, dictatorial regimes. In the 1980s, the Soviet policy of glasnost or openness "was meant to let the damp vapours out of the basement, but it ended up destroying the system, as people began to think: 'What have we been doing all these years?'"

In happier circumstances, however, MacMillan believes that she and her fellow historians can perform a salutary role.

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"History protects us from ideas about national essences. I like to think we've punctured the myth of the eternal, unchanging nation. We are all, in some sense, the product of waves of immigrants. Some of what we think of as (typically) English is very recent."

The crucial question, though, is whether we can learn from the past in trying to cope with the challenges of the present. And this takes us back to earlier stages of MacMillan's life and career.

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She brings an interesting and probably useful background to her research, particularly to Peacemakers, her book on the conference that ended the First World War. Although she was born and brought up in Canada, she is the great-granddaughter of British chancellor and prime minister David Lloyd George, and she got many intriguing insider glimpses into political life on her frequent visits to England.

"I knew my grandmother, a lifelong Liberal, and my great-aunt, who became a Labour MP," she explains, "so my family here was quite political and used to talk about politics a lot. I'd go and stay with my great-aunt in London and she got me tickets to the House of Commons. She would tell me the gossip and take me to dinner with (Labour politician) Dingle Foot. It was important and interesting, but it was still people doing it, who behaved much like people in other situations - they had friends, allies, issues, enemies."

As a result of these experiences, MacMillan has never been tempted either to idolise politicians or to take the easy option, common among her students, of blanket contempt. It also means that writing about the Paris Peace Conference inevitably touched on her family history.

"I didn't tell my publisher that I was Lloyd George's great-granddaughter because I didn't want to be seen as someone who was doing it out of family motives," she says. "If anything, I was inclined to be more severe on him than on the others. I'd read all the stuff about the devious Welsh wizard who had no moral core. I probably started out by agreeing with it.

"But what happened as I wrote the book was that I understood he was a greater negotiator than I'd ever realised. He was very quick on his feet. He had good instincts and tremendous energy. He didn't get tired. He'd occasionally get depressed, and bounce back. In the end I came away with a different view - I hope as a historian and not as a great-granddaughter."

Writing about 1919 also gave MacMillan a strong sense of how history gets entangled with the present. She struggled to interest publishers in Peacemakers - and one pompously informed her: "Nobody wants to read a book about a group of dead white men sitting round a table drawing up peace treaties."

Events soon proved this notion absurdly myopic and parochial (while the book became a great success for another publisher and the winner of a clutch of awards, including the Samuel Johnson Prize in 2002). The end of communism recreated the map of the world, particularly East-Central Europe, and brought back to the surface many long-forgotten "nationalist narratives". The fragile settlement after the First World War offers many crucial precedents and warnings, MacMillan observes.

"The Cold War was very simple: there was a good side and a bad side, and most of us assumed it would just go on for ever," she says. "Suddenly we were in a different world - and what do we have to guide us? We've got these historical enmities mattering again. We've got to figure out what to do next and one of the things we can do is look back and ask: is there a similar situation in the past that helps us understand?"

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Outside Britain, the emotions generated by 1919 remain very much alive.

"I think every Hungarian schoolchild knows about the Treaty of Trianon (which drastically reduced the size of the country). I gave a talk in Budapest and tried not to get into it because it's a nationalist issue, so (then) there was a newspaper headline 'Visiting professor refuses to condemn Treaty of Trianon'." This is not the kind of headline one would be likely to find in The Oxford Times.

Politicians and pundits will always use history to justify their claims. The fighting that followed the American decision to commit troops to Vietnam in 1965 was preceded by a battle of the analogies. British attempts to appease Hitler at Munich, French setbacks in Indochina and the Korean War were all cited as the essential parallel that made the case for going in or staying out.

Some of this is inevitable. "We all use analogies," says MacMillan. "We have to. We have a huge thicket of information and we chart our way through it by using analogies."

Although none of them is ever perfect, they are pretty much all we have got. "What else can we do to guess the outcome of a particular course of action? Go to the astrologers? That is why business and the military study past battles and case histories."

The problem, of course, is that analogies get used opportunistically or simplistically - which is where those "intensely irritating" historians can step in to complicate matters. If they hope to influence policy, they have to strike a delicate balance and add nuance to decision-making - without numbing people with detail.

Yet MacMillan believes that serious historical understanding of the Great Depression, for example, could usefully feed into our responses to the current financial crisis, and the same applies to debates about going to war.

"The best thing history can do is inculcate a sense that we need to ask more questions," she argues. "Was Munich as everyone says it was? Is it always wrong to try to meet people's demands? When we use the term 'appeasement', which we always do in a condemnatory way, are we prejudging the issue? 'We have to get rid of Saddam, because otherwise it's appeasement.'

"In relation to Iraq, many people said at the time that the right analogy was not appeasement but containment: 'Saddam Hussein has been contained, he is certainly a threat to his own people, but he's not a threat to his neighbours or the wider Middle East'.

"If you do something to try to keep China happy, is that 'appeasement' or 'statesmanship'? It all comes down to what label you give it."

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