Five years ago this week most readers of this newspaper were making plans to go on a demonstration. More surprisingly, just as many Daily Telegraph readers were getting ready for the same event. For most of those who marched against the Iraq war on February 15 2003 it was the first time they had ever demonstrated for or against anything in their lives. It was a protest such as Britain had never seen before, all-embracing in its diversity and imposing in its unity of purpose.

While there are always arguments over the size of demonstrations (the 2 million-or-so figure we claim is supported by considerable polling and photographic evidence), there is no dispute that this was not merely the country's biggest political protest, but the biggest by a substantial order of magnitude.

Two things are obvious about the demonstration to "stop the war". First, the millions on the march were right. Not just right on balance, but right on every single aspect of the question. There were no weapons of mass destruction, Iraq did turn into a bloodbath, the invasion did not help resolve the crisis in the Middle East, and it did damage the cohesion of our own society and imperil our civil liberties while not making us one whit safer from terrorism. So the people were smarter than the politicians.

Second the demonstration did not stop the war. Our hope had been that mass protest could drive the British government out of its aggressive alliance with Bush and that the latter, isolated internationally as a result, would come under intensified domestic pressure. We came very close, as Donald Rumsfeld made clear. In the wake of February 15, Washington told Blair he could stand down our army if he wanted to.

The prime minister ignored that offer and the people he represents alike. However, failing is not the same thing as making no difference. February 15 has cast a long shadow over British politics since, and contributed to Blair's departure from office under circumstances - in public odium and with an exasperated party - scarcely of his choosing. What war have we stopped? The next one, perhaps.

The demonstration was the apex of a broader movement which touched almost every part of society in 2003. This included the greatest-ever engagement of British Muslims in active politics, thousands of school student walkouts, peaceful civil disruption in towns across the country, local authorities coming out against the war, and train drivers declining to move munitions for the invasion.

It was a movement entirely outside the established structures which normally mediate the relationship between people and power. It was organised by the Stop the War Coalition (with CND and the Muslim Association of Britain as our partners), a campaign not 18 months old and run on a shoestring.

Hundreds of thousands of trade unionists joined the demonstration, while the TUC - its eyes on its ministerial connections, not its members - maintained a frigid indifference. Labour and Tory party members protested against their leaders, while Liberal Democrats dragged their hierarchy to the demonstration behind them. Marching at the head of the demonstration, I missed what may have been the most telling sight of the day - Piccadilly blocked by people without a single banner among them. This was the march of the unmobilised.

It was also a march against Murdoch and his mendacious press, exploding the myth of his political omnipotence. Rupert said war, the people said no. All Alastair Campbell's strategy of controlling opinion through appeasing the Sun in vain!

The demonstration, and the movement around it, exploded the notion that society is slumped in a consumer-sodden apathy, and incapable of political engagement. The country's biggest mass movement followed a general election with the lowest turnout in modern times, and preceded one in which participation was scarcely improved. The problem is the system, not the people.

So perhaps the biggest lesson of February 15 is that it embodied the failure of representative democracy. It highlighted a gap between the electorate and the elected, a gap several hundred thousand lives have slipped down as a result.

The anti-war movement has lived under the shadow of that immense mobilisation too. But it was followed the next month by the biggest demonstration against a war British troops were actually fighting, by the biggest-ever weekday march (against the Bush visit to London later in 2003), by an unprecedented movement of military families against the war, and by a dozen further marches - including one which will mark the fifth anniversary of the war itself, on March 15. Opposition to empire has been put at the heart of politics as never before.

Emily Churchill, a Birmingham school student at the time, described the experience as "trying to steer the course of our country with our own hands". Of course in 2003 other, American, hands were on the wheel. But the lesson of February 15 is that we can and we will.

· Andrew Murray has been chair of the Stop the War Coalition since 2001

office@stopwar.org.uk