The US was then isolationist, largely uninterested in Europe. Stalinist Russia was isolated for other reasons. Britain had to take the concerns of a world empire into account. France was petrified about the growing danger on the other side of the Rhine. The threat was indeed in the very heart of Europe, and unmistakably real. Britain's very existence was at stake. No weapons inspectors were needed to see whether Hitler was building "weapons of mass destruction". Everybody knew he was doing this illegally even before he openly announced it. He then used military might and bullying tactics to force changes to state borders within Europe. The annexation of what was left of Czechoslovakia in 1939, without any pretext of uniting ethnic Germans, finally convinced the government to take a stand, at the risk of a war they did not want.

Today, there is no self-evident threat from Iraq. There is no invasion of a sovereign territory (as in 1991) to repulse. We have to take it on trust that Saddam is building weapons of mass destruction. Even if he has them, he is unlikely to use them against Britain or America - seemingly bent on war and towing Britain in its slipstream. The tanks at Heathrow are not there to fend off an attack from Saddam. But we can't destroy the invisible source of that menace, which is likely to grow, not diminish - fostered by a war for which the reason is far from plain. In 1939, the reason was all too obvious.

· Ian Kershaw is professor of modern history at Sheffield University.

Mark Mazower
In 1939, the Third Reich was the most powerful and highly armed state in the world. To defeat it took six years, even though for much of that time it was fighting on several fronts at once. But although the Nazi party was destroyed, Germany itself was not: divided and occupied for half a century, its essential unity re-emerged with the collapse of the Berlin Wall. Iraq, by contrast, weakened by defeat in 1991 and sanctions since, is so far from being the most powerful state in the Middle East that, even now, hostile forces control its skies and northern territories with impunity. A war with it is likely to take much less than six years but coping with the aftermath will make dealing with postwar Germany look easy.

In 1956, the US opposed war and forced Israel to withdraw from both Sinai and the Gaza Strip by threatening to cut off aid to it entirely. Eisenhower's anti-colonialism is now a distant memory. Today, the US imports half of its total private consumption of oil, and believes toppling Saddam will help it secure this. Will the people of Iraq thank Washington for their liberation? Maybe. But unless a miracle happens and the US - there is no one else - forces Israel to allow a viable Palestinian state, a new Iraq war will also feed terrorism, not starve it. As for weapons of mass destruction, this war may get rid of Saddam's. And then? War on North Korea next? Or Iran? We hear a lot at present about the limits of diplomacy and the virtues of military force. True statecraft appreciates that force has its limits, too. Hitler, the messianic leader of a rising power, never understood this; Churchill, prime minister of a waning one, did. Which role model for our neo-Churchillians now?

· Mark Mazower is the author of Dark Continent: Europe's Twentieth Century (1998) and The Balkans (2000). He is professor of history at Birkbeck College, London.

Andrew Roberts
This is not another Suez crisis, for the obvious and straightforward reasons that the west is not today trying to recapture anything for itself, that Egypt posed no military threat to the Nato allies in 1956 and that the British government is pursuing its ends openly through the UN, at least initially, rather than through collusion. Moreover, the people of Egypt were fully in support of Nasser, whereas the moment a US-led invasion of Iraq is successful, the full extent of the Iraqi people's fear and hatred of Saddam will immediately become evident.

No, the situation is far closer to the late 1930s, when a fascist dictator stealthily acquired weapons of mass destruction - the Luftwaffe's bombing arm - and attempted to acquire nuclear weapons, too. That totalitarian dictator later invaded his neighbour (as Saddam did), gassed his political and racial enemies (as Saddam has) and brutalised and tortured his own people (as Saddam does.) The League of Nations, on the morning after Poland was invaded, had on its urgent agenda the standardisation of European railway gauges. Today's United Nations is fast shaping up to be equally ineffectual.

In the face of a danger that the left, the Church of England, much of the establishment, the press and the French denied really existed, a lone voice told the truth unashamedly again and again until events forced the rest of the nation to listen. This brave politician faced public obloquy and collapsing political popularity, until he was proved right, when he became the most popular prime minister in recent memory. For Churchill, this apotheosis came in 1940; for Tony Blair, it will come when Iraq is successfully invaded and hundreds of weapons of mass destruction are unearthed from where they have been hidden by Saddam's henchmen.

Rarely in history are we allowed quite so exact a template as we have been given militarily by what happened in the Gulf in 1990-91, and politically by what happened in Europe in the 1930s. But this time, were the west not to act and Saddam eventually to build nuclear bombs, he would have more destructive capacity even than did Hitler.

· Andrew Roberts's new book, Hitler and Churchill: Secrets of Leadership, is published this week. The accompanying TV series starts on BBC2 on March 7.

Simon Schama
I don't think it's a case either of 1939 or of 1956. I'm allergic to lazy historical analogies. History never repeats itself, ever. That's its murderous charm. The poet Joseph Brodsky, in his great essay A Profile of Clio, wrote that when history comes, it always takes you by surprise, and that's what I believe, too.

It is not 1939 because Saddam Hussein is not a rolling juggernaut of confident invasion and annexation (although he would probably like to be). Nor is it 1956 because the US is at the clumsy beginning of an imperial career, not the pathetic end of one.

There are two complicated modern problems that make this present situation extremely dangerous but unique. The first is in the shape of Islamic fundamentalist terrorism, where you have a movement that hates modernity but is equipped with hi-tech modern weapons. In the past, where you have had a culture that resists modernity, they have had primitive weapons. This is a rich man's terrorism with rich man's toys. Osama bin Laden is a capitalist of death.

The second issue is that, even though it is a struggle to prove a direct link between Bin Laden and Saddam, there is a kind of pond of availability of very nasty chemical and biological weapons. In 1939, you had the spectacle of the German army marching into Austria and Poland with tanks. Then, in a sense, all you had to do to oppose Hitler was meet his tanks with yours. With these weapons, the threat is less familiar, less visible, less clear - but you still need to drain the pond.

Finally, the 1939/1956 controversy does not move the argument on: there has been extraordinarily little debate about what the postwar settlement would be. Anyone could fight this war; it will be easy to win. But no one in the US or UK seems really to have any idea of what to do afterwards: what kind of regime there will be, who to protect and who to do the protecting, what legitimacy a new government will have, and so on.

As a consequence, if you were Bin Laden, you would be thrilled about the prospect of war: either there will be a great fat target of a western presence in Iraq for several years or there will be a broken and chaotic state: either way it will be a teddy bears' picnic for terrorism.

· Simon Schama is professor of art history and archaeology, Columbia University. He is working on a book about arguments in Anglo-American histories, entitled Rough Crossings.

Linda Colley
For connected reasons, neither 1939 nor 1956 offer appropriate parallels to a projected second Gulf war. Saddam may in essence be as evil and megalomaniac a man as Adolf Hitler (how would one judge?). He is certainly a dictator who has killed large numbers of people. But, as a determinedly secular ruler, he lacks the international ideological underpinning fascism gave Hitler. And, crucially, he lacks comparable hardware. In 1939, Germany had the strongest, most modern army, navy and air force in the world. In 2003, it is Iraq's primary enemy, the US, that unquestionably possesses the world's greatest stock of weapons of mass destruction.

Nor is Suez a useful parallel. To be sure, Colonel Nasser was seen by the French and British as an uppity Muslim leader who did not know his place. The British had demonised Tipu Sultan of Mysore in the 1790s, and the Mahdi in the Sudan in the 1890s for much the same reason. Worrying about renegade Islamic leaders is an old western imperialist tradition. But the British were bound to fail in 1956, because Washington had set its face against them. Two thousand and three will be different. If we invade Iraq, it will be on the coat-tails of the Americans. And they will not fail.

But the vital reason why history in this case offers limited help is that, since September 11, 2001, we have entered a different world in terms of both danger and paranoia. In the past, terrorists killed hundreds at most. Now we know they can kill thousands; and that they may in the future kill millions. Whether invading Iraq will make this more or less likely is the question that really divides us, and to which we possess no answer. All I know is that I look at photographs of ordinary Iraqis caught between the rock of a foul ruler, and the hard place of approaching Armageddon, and it breaks my heart.

· Linda Colley is Leverhulme research professor at LSE. Her latest book is Captives: Britain, Empire and the World, 1600-1850

Eric Hobsbawm
The war which is likely to break out shortly is not like the second world war. All comparisons with appeasement and Munich are so much hot air which merely justifies starting a war. No historian will believe them for a moment.

This is not a war against an aggressor, still less against one capable of overrunning Europe. It is not in any sense a war of defence but of aggression by the greatest military power on earth against a smallish, though very nasty, dictatorship. It's a war which the US wants and nobody else in the world wants, except the government of Sharon and the UK cabinet. If the US did not insist upon it, nobody would have proposed it or thought about it. Even today, the neighbours of Iraq are extremely reluctant to take part in it.

It is not like Suez either. Suez in 1956 was the last throw of two declining imperial powers, plus Israel, which, then as today, has its own agenda. It ended in humiliation for Britain, not for military reasons but because the US pulled the plug on us.

The US will win the new Iraq war. There is no real doubt about that. They won it quickly in 1991 and there's no reason to suppose it will not be won quickly again - even though the US remains extremely reluctant to get any one of its citizens killed. Whether this will work in a few days, nobody can tell.

And then? Will the US stay? Unless they are prepared to stay as long as they did in Japan, Germany and Korea, they cannot guarantee stability. Have they any clear idea of what they want to happen with Iraq and the Middle East after the war? I can detect no serious plans but only a lot of editorialising about the benefits of multi-party democracy. There is not much evidence in the Balkans that this alone will be sufficient.

Finally, in short, the US, it seems to me, has used 9/11 to proclaim its global hegemony and its capacity to make other countries go along with it. I think they underestimate the complexities of the world; hi-tech means of mass destruction are not enough.

· Eric Hobsbawm is emeritus professor of history at Birkbeck College. His most recent book is his autobiography, Interesting Times: A Twentieth-century Life.

Richard Evans
History never repeats itself, so anyone looking for parallels between the present situation and past events is likely to be disappointed. Not that there has been any shortage of such parallels drawn in the past few weeks by politicians seeking to encourage their supporters or discredit their opponents. But all of them are specious in one way or another.

Nato's disarray over policy towards Saddam Hussein's Iraq has been compared to the League of Nations' impotence when confronted with Mussolini's invasion of Ethiopia in 1936. But the League of Nations was trying to stop a war, while Nato is talking about starting one. Mussolini's weapons of mass destruction included poison gas, deployed against the poorly-equipped armies of the Ethiopian emperor. But the case for intervention in terms of the real political interests of the leading member-states of the League of Nations, such as France and Britain, was feeble, given that Ethiopia was a miserably poor state of no strategic importance, and wars are fought over real political interests, not for reasons of morality.

It is easy enough to brand the opponents of an invasion of Iraq as "appeasers", but this is another specious parallel with the past. Britain and France did not declare war on Germany in 1939 because Hitler was maltreating his own people, but because Hitler invaded Poland, and because his invasion of Poland followed his invasion of Czechoslovakia earlier the same year. This was enough to convince most people who had thought he could be appeased by revising the 1919 peace settlement to include more Germans within the Reich that this would not appease him at all, since he was now taking over non-German parts of Europe at a rapidly increasing pace. Even so, the allies did not invade Germany in 1939. Instead, it was Hitler who ended the "phoney war" in 1940 by invading France, Belgium, Holland, Denmark and Norway.

Saddam is not Hitler. He has not invaded anywhere since the Gulf war and has shown no signs of wanting to do so. Possessing weapons of mass destruction is one thing, intending to use them quite another. There is no credible evidence to link him with international terrorism. He is not threatening to take over the world. His country does, however, possess economic and strategic importance. And given the fact that the Iraqi regime is a murderous tyranny that once, some time ago, invaded a neighbouring country, it offers a morally convenient and geographically tangible outlet for the frustrations of combating the real threat of global terrorism.

The Franco-British invasion of Suez in 1956 owed something to similar psychological factors, particularly in the case of the British prime minister Anthony Eden, who saw a Hitler in every little dictator and wanted to compensate retrospectively for his failure as foreign secretary before the war. But principally the French and British invaded Egypt because the Egyptian government had taken over the Suez Canal, which they had previously controlled, and which still seemed to them to be of vital strategic importance as an artery of seaborne communication with their remaining colonial possessions in Africa and Asia. The invasion was intended to recover the canal, not to occupy Egypt, and so was a very limited operation.

It failed because it was not supported by the world's leading superpower, then as now the US. The US is the driving force in the imminent invasion of Iraq and so the project is likely to succeed, at least in its immediate aims. Its wider consequences, however, are wholly incalculable.

· Richard J Evans is Professor of modern history at Cambridge University and the author of Telling Lies About Hitler (2002).

Avi Shlaim
The failure to stand up to Adolf Hitler is increasingly being invoked as a reason for standing up to Saddam Hussein. In a clear allusion to the appeasement of the 1930s, Tony Blair insisted that "all our history - especially British history - points to the lesson that if international demands are not backed up with force, the result is greater insecurity." But to compare Saddam with Hitler is to greatly inflate his importance and the danger he poses to international order. A much closer parallel is the Suez crisis of 1956. Anthony Nutting [foreign office minister in Anthony Eden's government], who resigned over Suez, called his book No End of a Lesson. Tony Blair would be well advised to ponder some of these lessons before embarking on another imperial adventure in the Middle East.

Eden thought that he was applying the lessons of the 1930s in dealing with Gamal Abdel Nasser and the result was a fiasco that brought his own career crashing down. Eden demonised Nasser, personalised the issues, and went to the length of colluding with France and Israel with the aim of knocking Nasser off his perch. The chiefs of staff had deep misgivings about the war. One senior officer exclaimed: "The prime minister has gone bananas. He has ordered us to attack Egypt!" Britain attacked Egypt without the authority of the UN and it was roundly condemned for its aggression. There is, however, one important difference between 1956 and the current crisis. Over Suez, the US upheld the authority of the UN and led the pack against the law-breakers. Today, the Bush administration is hell-bent on the use of force to topple Saddam, with or without UN sanction.

Blair would be taking a huge gamble if he ignores public opinion and joins George Bush in an imperialist war to oust the Iraqi dictator. The Suez war brought to an end Britain's moment in the Middle East. Eighteen months after the attack on Egypt, Britain witnessed the defenestration of her royal friends in Baghdad. A war on Iraq today could go badly wrong, result in heavy casualties, fuel terrorism and end up by destabilising the entire region. As the moment of truth approaches, Blair would do well to reflect on the lessons of Suez. Politicians, like everyone else, are free to repeat the mistakes of the past, but it is not mandatory to do so.

· Avi Shlaim is a fellow of St Antony's College and professor of international relations at the University of Oxford. He is the author of several books on the Middle East, including The Iron Wall: Israel and the Arab World (2000).

Paul Kennedy
Nineteen thirty nine it is not. There a country which had already unilaterally moved its armed forces across international borders three times (the Rhineland in 1936, Austria in 1938, and Prague in 1939) committed a further breach against Poland in September 1939, causing the British empire and France to fulfil their clear international obligations under the League Charter and their military alliances with Poland. War was formally declared. No one has said that the British and French actions were wrong; the criticism has always been that they should have taken place earlier. When Neville Chamberlain reluctantly declared war, even his strongest critics conceded that he had striven, for years, to avoid military action.

None of the above elements will be in place if the US and UK go to war without a further UN resolution. Having taken the matter to the security council last autumn - in clear recognition that the world community gave it extraordinary powers in matters of war and peace in 1945 - they cannot thumb their noses at the council now. There is no transnational aggression by Saddam Hussein this time around. There is no military alliance to fulfil. And the White House hawks give the impression that they are eager for war.

This would change markedly, of course, if the security council backs the US/UK position by a further resolution next month, declaring Iraq to be in breach and authorising all necessary measures to cease that. But if such a resolution is not forthcoming, then military action would indeed look like a Suez adventure, a folie de grandeur with strong possibilities of backfiring. President Bush models himself a great deal on Churchill; but the model in question might turn out to be Mussolini.

It is also worth noting that, when Gladstone's government intervened in Egypt in 1882 - to uphold "order" against Muslim, anti-western radicals - it claimed it would leave that country soon. As it turned out, Britain didn't leave for another 73 years.

· Paul Kennedy is professor of history and director of international security studies at Yale University. Author and editor of 16 books, including The Rise and Fall of the Great Powers, he is writing a study of how to think about, and use, the UN in years to come.

Michael Burleigh
Historical analogies are rarely useful, though there is a touch of the Jacobins about parts of the US administration seeking to bring democracy to the Middle East as a whole. We are in a weird world when James Woolsey, the former head of the CIA, says that the Saudi capital Riyadh reminds him of Versailles in 1788. These people think stability has got them nowhere, so why not see where instability leads? It is really not about oil, which Tony Blair correctly dismissed as the most absurd conspiracy theory he had heard. If President Bush and Vice-President Cheney were that interested in oil, they would not have divested themselves of their own oil business interests to pursue barely lucrative political careers.

The consensus line involves stopping Saddam from supplying terrorist surrogates with horrendous weapons, whose manufacture he will resume when the UN is distracted elsewhere. There are hawks, too, who wish to bring democracy to the Middle East as a whole, unaware of Robespierre's warning about the world resenting "armed missionaries". However that internal American debate works out, there isn't going to be a Suez-style debacle, but a war so fast and ferocious that it will be over almost before you know it has started, and it will be regarded as liberation by the people of Iraq.

· Michael Burleigh is Kratter visiting professor in history at Stanford University, California. His next book is on politics and religion since the French Revolution.

Norman Davies
I belong to the school that doesn't put much trust in historical precedents. They only show that no precedent ever fits exactly and that history never quite repeats itself.

None the less, it is fascinating to see how many politicians, from Rumsfeld upwards, are using their views on history to justify policies towards Iraq. Rumsfeld seems to think that Churchill advocated a pre-emptive war against Germany. And no doubt some Iraqi professor, at this very minute, is polishing his thesis about Iraq being the "poor little Poland" waiting to be attacked by the new Hitler and Mussolini.

I don't like the comparisons with 1939. The Third Reich was potentially a top-class industrial and military power, that was in a phase of dynamic expansion. If it had defeated the Soviet Union it would have been the strongest state in the world. Iraq is incapable of mounting a comparable threat. It is a third-rate power which has already been badly defeated and which does not possess the means to attack Europe or the US. Saddam is under suspicion because he may try to attack Israel, which has already attacked him.

Nineteen fifty six fits the present stand-off better than 1939 does. An upstart Arab dictator, who was likened by a British prime minister to Hitler, was threatening to destabilise the region by nationalising the Suez Canal. And a "Coalition of the Willing" made up of Britain, France and Israel decided to teach him a lesson. But there the parallel ends. In 1956, one of the two superpowers of the day refused to support the adventure and promptly put an end to it. In 2003, by contrast, it is the world's only superpower that looks hell-bent on leading the coalition; and there is little chance of it being stopped by a third party.

So what about 1914? The strongest military power in sight is made to feel insecure by a terrorist outrage. Instead of confining its response to the known source of the terrorism (Serbia), it lashed out at one country, which it suspected of abetting the terrorists (Russia), and then at another country (France), which was linked to the first. Then it lost the plot. Worst of all, it calculated that the war would be won by Christmas.

· Norman Davies is a fellow of the British Academy at Wolfson College, Oxford. He is the author of Europe: A History and The Isles.

Richard Overy
It doesn't look like 1939 or 1956. The effort to grab historical examples off the shelf and use them to legitimise what you're doing at the moment seems to me to be treating history irresponsibly. You cannot compare this with the past; this is a unique set of circumstances.

If you must have a historical analogy, my choice would be the Boer war, where you had a large, heavily armed imperial power trying to eradicate the threat from an awkward regional state which happened to control an important raw material. Attempts to cast the present as a re-run of the past do not work because Germany was a military superpowers in the 30s. This time around, all the superpowers are on the western side. Likewise 1956, because the US and UK are not fading imperial powers about to be disgraced, but very powerful states seeking a military success - which they will almost certainly achieve.

But I think a postwar settlement will be extremely difficult to broker. Again, if we need historical analogies, we need only remind ourselves of Britain's interventions in Iraq in the early 1920s and during the second world war that ended in humiliation.

To me, this is not about the moral highground Tony Blair has been speaking of. This is about the US retaliating for September 11, and the coming confrontation is really to do with American concerns rather than those of the international community.