"It's the economy, stupid!" This sentence, scribbled on the notice board in Bill Clinton's campaign office, became the soundbite of the 1992 US presidential election. It described the view that the most important thing in the political process was the health of the economy. So, although George Bush senior, the incumbent president, had just scored a military victory during the first Gulf War, it was never going to help him win the support of electors suffering unemployment, wage cuts and general economic misery.
There was nothing novel about the comment that the economy was central to the political fortunes of presidents and parties - hence the word "stupid". Ever since Aristotle, the Greek philosopher who tutored Alexander the Great and who penned Politics in about 335BC, the economy has been regarded as integral to the body politic.
In modern times, self-styled economists have sought to describe how and why the contemporary economy works. Adam Smith, who published his The Wealth of Nations in 1776, as American revolutionaries signed their Declaration of Independence from the British Crown, was the first great exponent of this.
If economists have endeavoured to comprehend the minutiae and the mechanics of the contemporary economy, it is historians who have answered the much bigger, more panoramic question: why do economies collapse?
It was ancient Rome that provided historians with the best case study for tackling the subject of political - and, implicitly, economic - collapse.
Livy and Tacitus were among the first but not the last to tell this story.
Sir Walter Raleigh addressed it in his History of the World , written in 1614 in the Tower of London while he awaited the executioner's axe. He noted that all great empires - from Babylon, through Persia, Egypt and Macedon, to Rome - were undone by the hubris and aggrandisement of princes.
In the following century, Edward Gibbon, a historian-cum-politician, published the archetypal study of politico-economic collapse: The History of the Decline and Fall of the Roman Empire , tracing the story of the ancient world's greatest power. It did not stop in 410AD, when the Visigoths sacked Rome, the conventional date for the end of empire, but continued to 1453, when the Eastern empire in Constantinople finally collapsed before the might of the Ottoman Turks.
British politicians, controlling an even bigger empire, took note. William Gladstone, Britain's Prime Minister four times between 1868 and 1894, ranked Gibbon among the three greatest historians of all time.
Yet Gibbon offered no pecking order of reasons for the collapse of Rome.
For him, "the decline of Rome was the natural and inevitable result of immoderate greatness". He wrote: "Prosperity ripened the principle of decay; the causes of destruction multiplied with the extent of conquest; and, as soon as time or accident had removed the artificial supports, the stupendous fabric yielded to the pressure of its own weight."
Historians have since tried to draw up a list of causes. Michael Grant, a modern populariser of the classical world, in The Fall of the Roman Empire: A Reappraisal, written in 1976, identified a list of 13 "defects" or "disunities" that combined to bring Rome down. But if Gibbon had no such list, he did offer an alternative way of answering the question of collapse, turning it on its head. "The story of its ruin is simple and obvious," he wrote, "and instead of inquiring why the Roman Empire was destroyed, we should rather be surprised that it had subsisted for so long." This proved an influential way of tackling the problem. Eric Hobsbawm, addressing the fortunes of the British Empire, talked of "relative decline", even though the US and Soviet empires had taken its place as global superpowers.
In his The History of the World (1976), John Roberts said that fifth-century writers bewailed the "collapse" of the Roman Empire so much that "it is easy to have the impression... that the whole society fell apart.
This was not so." In fact, he said, what seized up was the apparatus of the state. He suggested this happened because it "became too big for the demographic, fiscal and economic base which carried it", just as Gibbon pointed to internal decay as a cause for the collapse.
Eighteenth-century thinker Georg Wilhelm Friedrich Hegel made the same point. In The Philosophy of History , he presented the first great "universal history", starting his story in China and taking the reader through the Greek and Roman worlds, finishing with modern Germany. Each civilisation handed over the baton of history to the next, in a one-way path to the present day, as they buckled under the pressure of internal problems. He cited the example of the medieval city, which fostered merchants who would become the standard-bearers of the capitalist economic system that would replace the old feudal system.
Karl Marx, Hegel's most famous interpreter, went one step further, pointing to the seeds of decay in a capitalist society. In Das Kapital , he envisaged the capitalist state, in which a privileged minority of property owners predominated, being replaced by an uprising of the marginalised working class.
In the 20th century, there was a dramatic rise in the number of people attempting a world or global history and, in doing so, seeking explanations for the collapse of economies, empires or civilisations. At the end of the First World War, H. G. Wells started his The Outline of History , which covered the history of the world from primordial times to the present. His answer as to "why the Roman republic failed" was: "The essence of its failure was that it could not sustain unity."
As Wells put pen to paper, so too did Oswald Spengler, a German historian, whose Der Untergang des Abendlandes traced the "decline of the West" and contended that each culture was subject to the same laws of growth and decay. Arnold Toynbee, an English classicist, followed Spengler's example.
His A Study of History divided history into the stories of distinct peoples, each with a separate past and future. But he did think there were common causes of decline. He pointed to domestic weaknesses as the primary cause of collapse, not to the actions of barbarians or foreign powers.
In the second half of the 20th century, the pessimism of historians writing during the war years gave way to a new optimism. In 1963, William McNeill argued in The Rise of the West that contact with strangers was the major engine of social change. In describing the decline of the British Empire, he observed that "if it had been left to its own devices", industrial revolution would have "died down". But Britain's industrialism, "reacting with divergent attitudes and institutions in other lands", acquired a second impetus, and the operators of the old empire "like everyone else in the world, had to adjust themselves accordingly".
Meanwhile, the US had assumed a new primacy, becoming, by 1917, "a world power", eclipsing Western Europe as the undisputed centre and arbiter of Western civilisation. This has led historians to look for the seeds of the decline and fall of the American empire. In The Rise and Fall of the Great Powers: Economic Change and Military Conflict from 1500 to 2000 , written in 1988, Paul Kennedy highlighted a conundrum that has exercised economists, politicians and historians since classical times: to be a great power demands a flourishing economic base, yet by going to war (or at least, devoting resources to armaments) a great power risks eroding that base, especially in relation to others that devote their resources to productive investment for long-term growth. Given this, he predicted that the hegemony of the US, the Soviet Union and other major states would erode as other nations grew wealthier.
This thesis, which alarmed Washington strategists, has already proved half true. The Soviet Union is no more, having engaged in a costly Cold War with the US. Meanwhile, economists, citing historical precedent, are drawing up scenarios for the day when China eclipses the US. Goldman Sachs, the US investment bank, in a recent report for investors, declares that China will overtake the US as the number one nation in 2040. The reason for this will be less about US collapse and more about Chinese catch-up. In so far as there is an answer to the question, "Why do economies collapse?", this point about relative decline and the growing burden of an expanding empire would seem to have been the historians' most popular response.
Simon Targett is a journalist on the Financial Times .