Why do wars begin? Peter Furtado looks at the facts, while the ancient Greeks, Tom Palaima says, could have warned us that peace is an illusion

The word "war" claims a certain dignity to its operations and obviously means more than "absence of peace", but it is not easy to define. Only a small proportion of history's large-scale outbreaks of violence began with some kind of formal declaration or even at an identifiable moment. Exactly when a raid, a vendetta, a rebellion - or a police operation - becomes a war often depends on who is doing the defining, and why. Some, like historian John Keegan, see the "western way of warfare", with two clearly defined armies locked together in a killing spree, as normative, but there are many other forms.

Some wars are exercises in grabbing loot or land, pure and simple - and many cultures have made no apologies about this: the Germanic tribes that took over the Roman empire, and the Mongol hordes that built up the largest land empire the world has ever known, sought no further justification for their actions. But others - and this is particularly true in the case of wars fought by settled civilisations with developed moral codes - in a tradition dating back at least to the writings of St Augustine in Late Antiquity, felt it imperative to define a "just war" and assert the justice of the cause of the day before urging others to die for it.

Just as it takes two to argue, so it takes two to make a war. Hitler's invasion of Czechoslovakia in 1938 - however immoral - did not involve war, whereas the invasion of Poland the next year did because it was actively resisted, first by the Poles and then by their allies, France and Britain.

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The fact that both sides have to opt to fight explains why, so often, both sides can claim they are fighting defensively.

There has long been a pessimistic argument that conflict between humans is a natural condition. A variation of this is the Marxist belief in the inevitability of conflict between classes, and between the states that represent class interests. Such arguments pass the question of the origin of war back to the philosophers, theologians, anthropologists, psychologists and biologists. But it is historians who can account for particular outbreaks at particular times and answer questions such as why is warfare not permanent and universal? Why and how do wars end, why and how do people strive to organise affairs so that they can be avoided altogether?

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One explanation is that if, as 19th-century military strategist Carl von Clausewitz proposed, warfare is an extension of politics by other means, then logically politics is also a substitute for warfare, and so the profession of diplomacy aims to manage conflict without resort to violence.

Diplomats have a stake in maintaining the status quo, or allowing change in small, manageable steps, whereas warriors look for cataclysmic and sudden change. Wars occur when diplomacy fails to allow for the necessary steps to occur smoothly or quickly enough - and the reasons can be varied. A. J. P.

Taylor, in his work on the wars of the 20th century, argues that it might occur without any party having responsibility for the breakdown in order.

Not all breakdowns in international relations occur as a result of the decisions of the elite: some conflicts are simply too visceral to be managed rationally through diplomatic channels, perhaps the results of ideological, religious, nationalist or racist hatreds arising from popular feeling. Here there may indeed be a popular clamour for war, though it may be manipulated by those who stand to gain from the conflict. The balance between popular demand and internal politics is often debated: Fritz Fischer's study of the origins of the first world war, for example, saw the bellicosity of the Kaiser as a matter of the internal management of German public opinion in the years up to 1914.

The casus belli may mask underlying causes that reflect long-term shifts in the relative power of the opposing forces, perhaps as one develops economically or technologically more quickly than the other. Another way of looking at this is to claim that the wars are actually stimulated by economic conflict (the old Marxist approach) or technological conflict - whether a race to control resources or the opportunity given to one party by its overwhelming superiority. But the decline of historical determinism makes it clear that such factors cannot be considered alone, irrespective of the choices made by political and military elites.

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The fact is, the outbreak of a war cannot be explained purely in terms of the strategic needs of generals, or the desires of politicians, or the demands of bankers, or the allure of armaments, or the hypocrisy of the priesthood, or the blindness of the people, or even the wickedness of human nature, important though all those may be. Cultural context is also important, in particular that of the mindset and value systems of the leaderships. Some political systems are more likely to opt for war than others. The religious system that sustained the Aztec empire in the 1500s saw war and sacrifice as a path to honour, and the entire Aztec social and economic system was built on the fighting of regular wars that would supply the prisoners whose blood sacrifice could feed the gods. Some interpretations of Islam promise paradise to those who sacrifice themselves in a holy war: bellicose policy is more likely to be found in a state where such interpretations predominate. Conversely, democracies like to believe they are notoriously reluctant to go to war, at least with one another.

No single explanation of war-making can embrace Aztec and Panzer, Mongol horde and Wellingtonian regiment. It is more fruitful to consider the circumstances that make rational men and women consider fighting a worthwhile option. Using hindsight to explain this can only diminish the weight of the original moment of decision.

Historians have always been fascinated by wars. Thucydides and Xenophon saw war as the result of political calculation and shifts in the balance of power, although both considered the wars they described as cultural clashes between two distinct and ultimately antagonistic world-views - to Thucydides between the democratic Athenians and the conservative and oligarchic Spartans; to Xenophon between the imperial, oriental tyrannical Persians and the federal, freedom-loving, nationalistic and decent Greeks.

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To Roman historians Livy and Caesar, war was a natural function of the state, something justified by the very successes in Roman arms that they chronicled. The historians and chroniclers of the Christian Middle Ages, led by the Venerable Bede, saw history as having a didactic meaning, tending to see the suffering caused by war as God's punishment for wickedness and success in war as a sign of divine favour.

These two approaches, the realistic and the moralistic - supplemented by the structuralist approach that argues that wars are an inevitable result of fundamental contradictions in the system of power - have dominated discussion up to our own day. Plus, perhaps, the cock-up theory. While long-term causes were popular in the Marxistic 1960s and 1970s, they have since fallen prey to revisionism: for example, the English civil war was seen by Marxist historian Christopher Hill in the 1960s to have had long-term economic causes and deep intellectual roots in the transition from a feudal society to a commercial one, whereas today most historians blame it on short-term miscalculations and point out that no one foresaw it, even 12 months before hostilities broke out.

Not surprisingly, the two wars that have seen the most debate over their outbreak are the two world wars of the 20th century. While Fischer blamed the German high command for challenging British supremacy and destabilising the balance of power in Europe, others saw the war as resulting from a calculated risk by Germany that got out of hand; a third approach takes the focus away from Germany and blames the intellectual and cultural environment of Europe, while a fourth suggests the entire thing could have been avoided if the British foreign secretary had played his hand more subtly in the summer of 1914. Of course, these do not have to be mutually exclusive.

This argument has a direct bearing on attitudes to the Treaty of Versailles, which itself is often seen as the contributory cause of the rise of Hitler and the return of war in 1939. Indeed, some historians prefer to consider the two wars as part of a single conflict interrupted by a 20-year truce. But the fact that the two major wars of the 20th century were started by Germany led some to seek the origins of the war in the bellicose character of the German nation. For most, the second war was fought to end Hitler's plan of continental domination and to avert the consequences of the Nazi-Soviet pact.

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Fresh life has been breathed into all these questions by the war in Iraq, and historians have been as divided as any other group on its rights and wrongs. But they have probably been less noisy than in the debate on the "war on terror" in the aftermath of 9/11, when they debated the question of a historic "clash of civilisations" between Islam and the West, as Samuel P. Huntington had argued. The typical historian's counter to Huntington's assertions was a sceptical one, with an appeal to caution and complexity, and attention to the specifics of when, where, who and how.

Peter Furtado is editor of History Today .