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Blair followed 'masochism strategy'
The Iraq War was arguably the defining issue of Tony Blair's time in power, and the massive protests of February 2003 marked a shift in his style of leadership.
In building New Labour, Mr Blair paid obsessive attention to public opinion and was often accused of basing policy on focus groups and polls rather than principle.
But when hundreds of thousands of people took to the streets to urge him not to go to war in Iraq, he took entirely the opposite tack.
In the weeks that followed, Mr Blair followed a "masochism strategy" of putting himself up for questioning by hostile audiences, to drive home his message that he was convinced that Saddam Hussein's weapons of mass destruction posed a threat to the world and that no amount of opposition at home would divert him from dealing with it.
Mr Blair later spoke of how he changed during his decade in power, becoming a conviction politician who was less worried about being "liked" and more concerned about doing what he felt was right.
And there is little doubt that the experience of the Iraq War protests played a big part in that.
There is also no doubt that it fuelled the process of disillusionment which four years later saw Labour force out the most electorally successful leader in its history.
In times gone past, a Labour leader would have been expected to be at the head of an anti-war protest, and many activists felt betrayed by Mr Blair's refusal to listen to the demonstrators.
Although there were only a handful of resignations from the Government over the war - notably that of Robin Cook - and Labour went on to win the 2005 general election with Mr Blair at the helm, enthusiasm for his leadership was palpably waning among the party's grassroots supporters.
That disgruntlement encouraged MPs to turn towards Gordon Brown, the then Chancellor who had managed to avoid becoming personally identified with the decision to go to war.
For another party leader, the Liberal Democrats' Charles Kennedy, the February 2003 protests were a springboard to previously undreamed-of popularity.
Even though he was committed to opposing war only if the United Nations failed to agree a second resolution explicitly authorising military action, Mr Kennedy was invited at the last minute to address the crowds in Hyde Park.
There was deep concern among some of the Lib Dem high command about the wisdom of accepting the invitation, which put Mr Kennedy on a roster of speakers also including George Galloway and Tony Benn.
But his appearance cemented in many voters' minds the idea of the Lib Dems as the only mainstream "anti-war" party, winning them a new swathe of former Labour supporters.
At the 2005 poll, Mr Kennedy saw his party's share of the vote increase by almost 4%, picking up an extra 11 MPs, and it is arguable that Nick Clegg may have the Iraq factor to thank for putting him in a position to take his party into coalition with Conservatives after the 2010 election.
The Iraq War and the protest against it divided opinion and shifted allegiances in British politics.
It did not bring down a government, as it did in Spain, but its impact continued to be felt long after the military action was over.