Over the past few years, the image of George Bush as a Roman emperor, dressed in toga and laurel wreath, has been a hard one for his opponents to resist. But even after the bombing of the UN building in Baghdad, things have not gone as badly for him in Iraq as they did for the Romans. For 600 years, the dominant superpower of the ancient world sought to pacify the region - and all for nothing. The details of the various disasters that befell the Roman invaders are the stuff of Paul Bremer's nightmares. One emperor was killed; another, taken prisoner, was used as a footstool and then stuffed with straw. The most disastrous expedition of all, however, was the first, in 53BC, which led to the loss of 30,000 men and the head of its commander. This gory trophy was presented to an actor for use as a prop in a tragedy - the perfect illustration of how nemesis follows hubris.
The defeated general, a power-hungry plutocrat called Marcus Crassus, had sought to justify his Middle Eastern adventure in the name of principle and self-defence, but everyone knew that his only motivation had been greed. To secure the command, Crassus had been forced to twist arms, grease palms and pervert the constitution to a monstrous degree. As he left for the Middle East, a tribune of the people - the Noam Chomsky of his day, perhaps - had ritually cursed him. The Romans, a superstitious people, had long feared the temptations of overweening greatness. The news of Crassus's death came as no great surprise.
In time, the Romans would become more relaxed about their imperial role. Yet throughout the centuries of their rise to supremacy, while the shadow of their power was gradually spreading across the known world, they had affected distaste for the burdens of empire. Certainly, they had been reluctant to countenance the permanent occupation of foreign territory, and for much the same reason as their American heirs: provinces were seen as expensive and troublesome to run. Instead, Rome had preferred to exercise power indirectly, intervening where necessary against uppity tyrants, but then, once they had been safely overthrown, withdrawing her lethal soldiers home. Just so long as Rome's pre-eminence was humbly acknowledged, and visiting plenipotentiaries and weapons inspectors obeyed, Rome had preferred to leave foreign despots in place. In time, the inadequacies of this policy would become increasingly clear. The Romans, by hamstringing every regional power that might pose a threat to their interests, and yet refusing to shoulder the burden of direct administration, left the field clear for the emergence of terrorist states. Ultimately, the collapse of the Middle East into near anarchy stirred the Roman ruling class into constituting a formal empire and a system of provincial administration that would last for centuries.
In that sense, the establishment of a Roman empire was a mark of failure. It was only by subjecting the world to a centralised autocracy that its supremacy could be preserved. Such an autocracy was provided by the rule of the Caesars. Before then, Rome had been a polity of free citizens, a republic, the first and, until recently, the only one to rise to global power. The Romans had had no doubt as to the superiority of this form of government. Freedom based on the dynamics of perpetual competition had been seen as the Roman way. The proof of its superiority had lain in its trouncing of every alternative. "It is almost beyond belief," the historian Sallust boasted, "how great the republic's achievements were once the people had gained their liberty, such was the longing for glory which it lit in every man's heart."
Parallels between the US and imperial Rome may be tendentious, but not so those between the American and the Roman republics. When the Founding Fathers threw off the rule of George III, the ancient Roman watchwords of liberty and king-hatred were consciously adopted as the Americans' own. That is why, in a continent undreamt of by the ancients, a second Senate came to sit upon a second Capitol Hill. But if the civic virtues of the Roman republic were regarded as qualities to be emulated, its failings provided the Founding Fathers with a deadly warning. For the ancient republic, in the end, succumbed to its own greatness: as Rome had conquered the world, so her citizens had competed to become the masters of Rome until, in the final reckoning, not even the world itself was enough to sate their appetites.
"By now," Petronius wrote of the republic's last generation, "the conquering Roman had the whole globe in his hand, the sea, the land, the course of the stars. But still he wanted more." Some, like Crassus, stretched out their hands and fell. Others, however, notably Pompey and Caesar, succeeded in winning resources for themselves beyond all the imaginings of previous generations. The consequence of such obscene power was that both men had the capability to destroy the republic - and duly did. Caesar crossed the Rubicon, and Rome collapsed into civil war. Out of the wreckage emerged a monarchy - the rule of Caesar's heirs.
The relevance of this grim story to the Founding Fathers' experiment with republicanism is all too apparent. The dilemma that it illustrates is one which confronts all democracies: how are citizens to be guarded from the excesses that are the fruit of their own freedoms? Just as the Roman republic, even as it revelled in its own greatness, came to dread it as well, fearing the jealousy of the gods, so western arrogance is mixed with anxiety. Boardroom excesses; the widening gulf between rich and poor; environmental degradation: wealth and power breed their own high costs.
Most dangerously of all, perhaps, they can alienate the governed from the governing class. To the great men of the Roman republic, with their immense wealth, their family connections and their influence over the law, elections had never been a matter just of garnering votes. To that extent, Bush's victory over Al Gore in 2000 was the most authentically Roman in American history. Yet it is on this side of the Atlantic, in France and Italy, that the most recognisably Roman style of politician has emerged. To use the law courts to cling to power, and then to have to cling to power to avoid the law courts - this was a scam familiar to Cicero and Caesar. The president of the French republic has been relying on his office to stave off prosecution for years, while the prime minister of Italy exploits his wealth and patronage shamelessly, adjusts the law to suit himself and bribes the electorate with lavish circuses.
One of Silvio Berlusconi's compatriots had no doubt as to the relevance of Roman history. "He who would foresee what has to be," wrote Machiavelli 500 years ago, "should reflect on what has been, for everything that happens in the world at any time has a genuine resemblance to what happened in ancient times."
There may be periods when this claim appears outlandish - but the present is assuredly not one of them.
Tom Holland's Rubicon: the triumph and tragedy of the Roman republic is newly published by Little, Brown (£20)