What did 15 February 2003 achieve? Four people who were there, including novelist Ian McEwan, remember the biggest protest in British history.

Tansy Hoskins remembers stamping her feet to ward off the bitter February cold after managing to join millions of people squeezed into Hyde Park 10 years ago today for the United Kingdom's largest-ever political demonstration.

As posters were thrown onto warming fires by marchers who had wound their way through the streets of London hours earlier, the case against waging war on Iraq was laid out by speakers, including left-wing activists and members of Parliament, artists, the former Liberal Democrat leader, Charles Kennedy, and Jesse Jackson.

As a child, her parents had brought her on anti-war marches, and by February 2003 she was already deeply involved in mobilizing fellow students at the London School of Economics. But Ms. Hoskins recalls realizing how the Iraq war protests were different from the protests she was used to:

“Normally on demonstrations I run into a lot of people I know, but on this one I didn’t meet a single acquaintance. That was when I knew how big it was,” she says of a 2003 march that organizers put at 2 million strong, though police put the total at less than 1 million.

Despite failing to prevent Britain from joining the US invasion of Iraq 32 days later, the demonstration itself left a lasting political and cultural legacy. A generation of younger Britons, including many who went on to immerse themselves in causes such the Occupy movement, were radicalized. And positions taken at the time continue to manifest themselves as fault lines, albeit fading ones, in the Labour Party, which was in power at the time.

The population as a whole remains divided too, although a poll today for the Guardian newspaper found that a majority of voters (55 percent) agree with suggestions that "the London marchers were right," because "a war sold on a false prospectus delivered little but bloodshed." And just 28 percent believe the marchers were wrong, on the basis that the war's achievement in "toppling the dictatorship of Saddam Hussein" eventually made the world a better place.

Hoskins, who went on to immerse herself entirely in efforts to oppose Western policy toward Iraq and other Western interventions elsewhere, says of the 2003 protest: “It was the biggest demonstration ever seen in British history, and added to that; it was worldwide so you got a sense that this was how you changed the world. It was a life changing experience.”

Polls have found that increasing numbers of Britons subsequently came to believe that going to war was wrong. The seeds were sown for greater distrust of government and a reluctance to countenance further boots-on-the-ground interventions.

This week’s commemorative coverage of the march in Britain has seen salvos fired at the usual suspects: former President George W. Bush and former Prime Minister Tony Blair.

As for why it was so large, there is relative agreement among opponents and supporters of the war on some of the unique factors in play at the time.

On top of weaknesses in the case for war as put forward by the Blair government, Bush personified European caricatures of the dangerous American, and in the wake of 9/11 fears were running high that reactions could ignite regional and worldwide conflicts.

Both sides take issue with the narrative that the march was an anomalous eruption of protest among a normally passive population – despite often using different examples of other events. They include a demonstration against the criminalization of fox-hunting months before the Iraq war march, as well as other anti-war mobilizations and more recent demonstrations over university fees and against austerity.

Even so, numbers on subsequent demonstrations such as one in London today against intervention in Syria, the ongoing war in Mali, and a possible attack on Iran have not come close to equaling the size of 2003’s demonstration on Feb. 15.

Elsewhere, a supposed ambivalence on the part of large sections of society toward the march was evoked in one of the better-known cultural representations of the event, novelist Ian McEwan’s best-selling 2005 novel “Saturday,” in which the 2003 protest forms an intermittent backdrop.

Depicting the arrival in a part of London of the novel’s protagonist, a middle class professional who ponders the rights and wrongs of the war, Mr. McEwan, writes: “The large gathering of humanity in the history of the islands, less than 2 miles away, is not disturbing Marylebone’s contentment, and Perowne himself is soothed as he dodges around the oncoming crowds and all the pushchairs with their serenely bundled infants.”

Participants such as Hoskins however, are in no doubt about what they believe the march’s legacy to be. “As an example of how to build a mass movement it has not been surpassed,” she says.

“Not only did it have the traditional left but also, for example, there was the Muslim community, sections of the Jewish community, Liberal Democrats, even some Tories.”

“What we needed to do was do it over and over again and have industrial action alongside it.”

A longer version of this article appeared in the Christian Science Monitor.