The 20th century, it was often said in the run-up to the millennium, had been a century of bloodshed and tyranny, with the Nazi genocide and Stalinist terror regularly paired as the emblematic twin horrors of the era. The modern school history curriculum reflects a similar perspective. But when it comes to the role of colonialism and its aftermath, British reactions are usually cloaked in embarrassment or retrospective pride about a legacy of railways and "good governance".
There is precious little acknowledgement of the relentless and bloody repression that maintained a quarter of the world's population under British rule until barely half a century ago. Nor is there much awareness of the hundreds of thousands who died in continual rebellions across five continents, or from forced labour and torture in prison camps such as the Andaman islands, let alone the ubiquitous racist segregation or deliberate destruction of economic prosperity in places like Bengal. It is less than 50 years since British soldiers were paid five shillings for each Kenyan they killed, nailed the limbs of Mau Mau fighters to crossroads posts and had themselves photographed with the severed heads of Malayan guerrillas. But - as with other former colonial powers, such as France and Belgium - there has been no public settling of accounts, no pressure for colonial reparations or for old men to be tried for atrocities carried out under the union flag.
One consequence of this national failure to face up to the reality of Britain's impact on the world has been a casual enthusiasm for a latter-day revival of the imperial project. What began as an almost playful attempt at historical revisionism by rightwing pundits on both sides of the Atlantic in the early 1990s has, since September 11, flowered into a chorus of full-throated calls for the US and its allies to move from the informal imperial arrangements of the postwar era to the imposition of direct "international colonial" rule on rogue states. The argument has been most forcefully advanced by the Oxford history professor Niall Ferguson, currently filming a television series on the history of the British empire. But his passion for a new imperium - restrained only by a fear that the Americans may not have the appetite for the task in hand - is far from unique. Among others pressing for a modern imperial renaissance are the novelist and critic Philip Hensher, who suggested a viceroy be appointed to run Afghanistan, while the polemicist Mark Steyn insisted that compared with the current system of relying on corrupt and dictatorial regimes like Saudi Arabia to protect big power interests, "colonialism is progressive and enlightened".
Such voices could be more easily dismissed as nostalgic mavericks were it not for the fact that they reflect a far broader emerging consensus in favour of intervention against recalcitrant governments, UN protectorates and the imposition of western norms through legal and economic restraints on national sovereignty. This is the "doctrine of international community", first championed by Tony Blair during the Kosovo war, with its echoes of the liberal imperialism of the 1890s, but expressed in a language of partnerships and values to appease the sensibilities of the age. Underpinned by that postmodern conceit of "humanitarian war", it reached its emotional apogee in the vision of a reordered world he held out to Labour's Brighton conference last month. And so long as it is dressed up in a suitably multilateral form, the new liberal imperialists are just as happy with international colonial rule as their blunter rightwing counterparts.
A UN trusteeship or other multinational occupation arrangement is of course exactly what is being prepared for the benighted people of Afghanistan as and when US "daisy cutters" and Northern Alliance warlords finally displace the Taliban from the rubble of Kabul and Kandahar. We know roughly what such a setup will be like, because UN protectorates - effectively administered by Nato and its friends - are already functioning in Kosovo, Bosnia and East Timor (in Sierra Leone, Britain has preferred to act unilaterally). In every case, the results have been dismal - most notably in Kosovo, where the occupation forces have failed to prevent large-scale reverse ethnic cleansing. We have in any case been here before. In the aftermath of the first world war, the League of Nations handed out mandates to Britain and France to prepare countries such as Palestine, Iraq and Lebanon for eventual self-government. On the 80th anniversary of the Balfour declaration - in which Britain promised to establish a national home in Palestine for the Jewish people without prejudicing the rights of the Arab inhabitants - it hardly needs spelling out that the long-term fallout was calamitous.
The roots of the global crisis which erupted on September 11 lie in precisely these colonial experiences and the informal quasi-imperial system that succeeded them. By carving up the Middle East to protect oil interests - as Britain did when it created Kuwait - and supporting a string of unrepresentative client states across the region, the western powers fostered first the nationalist and then the Islamist backlash which now threatens them. The claim of the American political class that the US was attacked because it stands for freedom and democracy is more or less the opposite of the truth. In reality, the rage driving anti-western terror is fuelled by the fact that the west continues to deny the peoples of the area the freedom to determine their own affairs - and has repeatedly intervened militarily across the region to enforce its interests since the end of formal colonial rule.
There is simply no reason to believe that what did not work and was rejected during the colonial era will be accepted if it is dressed up in the language of human rights, markets and the rule of law. The 19th-century imperialists did not, after all, sell themselves as exploiters and butchers, but as a force for progress and civilisation, bringing education, trade and religion to all - they even claimed to be defending women's rights. The anti-colonial storm that swept away western direct rule in the 20th century cannot be reversed. If the US and Britain are set on a continuing course of armed intervention, punitive sanctions and multinational colonies, that is a recipe for indefinite war.
Blair has led Britain into four wars in four years - against Iraq, Yugoslavia, Sierra Leonean rebels and Afghanistan. So far, British and US casualties have been negligible. But the likely costs are now rising. When British troops slaughtered the followers of the Mahdi in Sudan or the Muslims of northern Nigerian a century ago, the fighting was far from home and the colonial forces had overwhelming technological superiority. "Whatever happens," wrote Hilaire Belloc, "we have got the Maxim gun, and they have not." Retaliations for colonial atrocities in the metropolitan heartland - such as the attempted assassination in London of General Dyer, the man who ordered the 1919 Amritsar massacre - were rare. Now all that has changed. Since September 11, we have discovered that the empire can strike back.