The erosion of liberty. Four words sum up four years. Since the attacks of September 11 2001, we have seen an erosion of liberty in most established democracies. If he's still alive, Osama bin Laden must be laughing into his beard. For this is exactly what al-Qaida-type terrorists want: that democracies should overreact, reveal their "true" oppressive face, and therefore win more recruits to the suicide bombers' cause. We should not play his game. In the always difficult trade-off between liberty and security, we are erring too much on the side of security. Worse still: we are becoming less safe as a result

How different it all looked a few years ago, at the turn of the century. One American writer summarised the outcome of the titanic ideological struggles of the 20th century thus: "freedom won". Simplistic, premature triumphalism, perhaps, but the last three decades of the past century did see an extraordinary spread of freedom, from Greece, Portugal and Spain throwing off their juntas and dictators, through Latin America turning to democracy and velvet revolutions in the Philippines, central Europe and South Africa, right up to the toppling of Slobodan Milosevic. For lovers of liberty, history seemed to be going our way. In Britain, the advent of Tony Blair brought promises of constitutional reform and more freedom of information, as well as the writing of European rights guarantees into national law in the Human Rights Act. It looked as if we would become more free.

Then came the fall of the twin towers in New York - the true beginning of the 21st century. Ever since, we have been going either sideways or backwards, as we struggle to respond to a real threat. We got off on the wrong foot on the very first day. As America's former anti-terrorism chief Richard Clarke records, when George Bush was reminded of the constraints of international law on the evening of September 11 2001, the president of the United States yelled: "I don't care what the international lawyers say, we are going to kick some ass."

Kicking ass, as it turns out, meant not just the invasion of Iraq but also Abu Ghraib, Guantánamo and, it now emerges, probably other secret prison facilities where people were held, and tortured, in a lawless limbo. Vice-president Cheney is reportedly fighting hard to exempt the CIA from a law, proposed by the conservative Republican and former prisoner of war John McCain, that would ban all American forces and agencies from using torture. At home, the USA Patriot Act allows routine invasions of privacy and curtailments of civil liberties that would never have passed before September 11. The words of America, the Beautiful - "Confirm thy soul in self-control/Thy liberty in law" -seem to have been forgotten in the "global war on terror"; or, as Bush put it, in kicking ass.

Unfortunately, this country, which was a beacon of liberty before the US was even invented - if you doubt this, read Voltaire's letters about his time in England, published in 1734 - has followed suit. After the entirely justified invasion of Afghanistan, we gave a patina of international legitimacy to the unjustified invasion of Iraq. There our own armed forces seem to have been reduced, in circumstances of extreme duress, to some practices of which we can hardly be proud. At home, we have seen successive tightening's of the anti-terrorism legislation - or, to put it another way, successive erosions of the Human Rights Act, and of other, older individual freedoms secured by common law, such as habeas corpus. This culminated in the proposal that terrorist suspects should be held for 90 days without charge. Legislation to outlaw the "glorification" of terrorism and a misguided attempt to protect Muslims by criminalising an ill-defined "incitement to religious hatred" both threaten free speech. And so we find ourselves in the surreal position of depending on unelected lords, and the Conservatives, for the defence of our liberties.

Meanwhile, across the Channel, France has just extended the applicability of a "state of emergency" from 12 days to three months. The direct cause is different, but the effect is also an erosion of freedom. The interior minister, Nicolas Sarkozy, is threatening to send rioting youths back to their "country of origin" - even though they have never lived there, may not have anyone to look after them there and may not even speak the language. The French detainees from Guantánamo were brought back, only to be locked up again in France. The French republican banning of the Islamic headscarf in schools is another, relatively mild but symbolically important - and, in my view, wholly counterproductive - infringement of an individual freedom.

So in all three classic lands of western liberty, America, France and Britain, we have witnessed an erosion of liberty. Of course, we should not be naive. As we saw in London on July 7, and before that in Madrid and Bali and New York, these are new and horrible kinds of threat. As the liberal philosopher Isaiah Berlin always reminded us, we cannot have all good things at once. We have to make trade-offs between desirable public goods, and the trade-off between liberty and security is one of the most basic in all politics. The totalitarianisms of the 20th century promised more security in return for less liberty. In liberal democracies, we generally accept less security in return for more liberty.

Faced with jihadist suicide bombers, we must reconsider and perhaps adjust the balance. Irritating though they are, I assume that tighter security controls at airports, railway stations and public buildings are necessary. Unlike many liberals, I also think identity cards may help, provided (and this is a big "if") they work properly and we have effective controls over the information stored on them. When I read that MI5 are recruiting 800 more spies to combat the threat of Islamist terrorism, I am disturbed - but I can see the argument for it. But in every case, we need to be convinced that the reduction of liberty will bring a commensurate increase in security.

What is unforgivable is the measure that makes us at once less free and less safe. Lately, we've been getting too many of those: actions designed to prevent suicide bombers that end up creating more of them. Delivering the Isaiah Berlin lecture in Oxford the other day, the American philosopher Allen Wood observed that "the death sentence is no use against suicide bombers". This was not just a somewhat black philosophical joke; it also contains a deeper truth. As Sir Ian Blair, the commissioner of the Metropolitan police, has just reminded us, the larger challenge for policing, but also for post-9/11 western policy altogether, is to help to create conditions in which people don't become suicide bombers in the first place.