There had always been a degree of unease about the prospect of any war with Iraq. In late 2002 and early 2003, as the Iraqi crisis rapidly rose up the agenda, most political debate and news coverage was restricted to questions about the progress or otherwise of the UN weapons inspector Hans Blix and his team, and to what this or that UN resolution actually meant.

By the time the war began on 20 March 2003, the legitimacy of US foreign policy was at stake, there was an awareness of the double standards in our own UK foreign policy and, to a lesser extent, debates about imperialism were on the agenda for the first time in a generation. We have, to a certain extent, the 15 February protest to thank for that. It was one of those rare moments in British history when the radical left had some palpable impact on the course of political debate.

That is not to say that all of us who went on the march (yes, I was there) would identify with the radical left, or that all the marchers thought of their opposition in anti-imperialist terms. But it is no secret that the hub of the movement, in the form of the Stop the War Coalition, hailed from that end of the political spectrum.

The success of the march, in terms of the number and diversity of people on it, led to some surprisingly positive press coverage. In contrast to most demos, these marchers could not all be dismissed as a bunch of Trots and Sixties throwbacks. It also allowed leading figures from the movement, such as Tony Benn, George Galloway and Lindsey German, the national convenor of the Stop the War Coalition, to appear on Question Time and Newsnight, and appeared to embolden the left of the Labour Party to speak out more vigorously against the war.

Looking back, I can distinctly recall the sense of celebratory optimism on the day. “They can’t ignore us now” was a common refrain. In fact, the march did more than that: it had leading figures of the government seriously worried, right up to the prime minister. If Alastair Campbell’s Diaries are to be believed, Tony Blair confessed to having slept uneasily that night.

Yet if they couldn’t ignore us, they could still override us. The bonhomie in the anti-war movement disappeared once the war began. Many who marched on 15 February have probably never been on another march since. “What would be the point?” they might reasonably ask. That, however, does not mean that the movement should be dismissed as a flash in the pan. The preparations and promotion began months beforehand and in some sense the movement carries on until this day. But when 20 March came around and it became apparent that we had not succeeded in stopping Blair from sending British forces into battle, the debate – and, indeed, argument – turned inwards: what could and should we have done differently?

Some people, many of whom identified themselves as anarchists, argued that we had picked the wrong tactics and that rather than relying exclusively on a peaceful march we should have engaged in far more civil dis­obedience. In reality, anything beyond the occasional publicity stunt would have been a disaster for the movement. To have blocked off all the bridges across the Thames in London, as proposed by the “autonomous” strand of the movement, would have caused nothing more than huge inconvenience for the public and would have alienated Britain from the anti-war cause.

In my opinion, what we needed more than anything else was an answer to the dilemma of what should have been done about Saddam Hussein and the appalling human rights abuses that were undoubtedly going on inside Iraq. Questions about this came up a great deal at public meetings, when leafleting the high street and in letters to local and national newspapers from supporters of the war. When asked about Iraq now, Blair always plays this card because he knows that opponents of the war don’t have an answer to it. If being on the left means anything, it ought to mean standing up for the oppressed. It shouldn’t have been beyond the wits of those speaking for the movement to have woven an answer to the problems of human rights abuses by non-western regimes into the fabric of their anti-imperialist principles. My view is that, just as we had weapons inspectors in Iraq, we should also have had human rights inspectors there. That would have done a lot to wrong-foot Blair et al.

If the march fell a long way short of achieving what so many of us desperately wanted, it wasn’t a complete failure either. This was the day when the message finally got through to Blair and the Conservative opposition that their war was going to be profoundly unpopular. The decades-old assumption that the British public was instinctively hawkish and suspicious of any politician who doubted the efficacy of going to war came to an end on that day.

Blair failed to realise this in time (if he ever came to realise it). His reputation has never recovered. David Cameron would be wise to take that on board as he ponders his options over Syria and Iran and in North Africa.

Ian Taylor is a lecturer in media and communication at the University of Leicester