Almost exactly a decade ago two million people responded to the call by the Stop the War Coalition to march against the illegal invasion of Iraq.
A similar scale of mobilisation is required now to defeat the government's Bedroom Tax, which is set to affect hundreds of thousands of poor people next month.
The first co-ordinated response to this vicious act of ruling-class politics will take place in dozens of towns and cities throughout Britain today when local groups will draw attention to the injustice of the tax.
Just as the war in Iraq was justified on the basis of lies relating to non-existent weapons of mass destruction, so Work And Pensions Secretary Iain Duncan Smith's legislation is premised on a falsehood.
Duncan Smith would have us all believe that families with children are denied council accommodation because smaller families are selfishly occupying houses and flats too big for their needs.
Households with designated "surplus" bedrooms that are in receipt of housing benefit will find their income docked by £14 a week from April 1 for a single extra bedroom and £25 for two such rooms.
The falsehood propagated by Duncan Smith and his cheerleaders is that these people can transfer to council accommodation more in tune with their needs.
This fantasy declaration was compounded by David Cameron's equally misleading claim that families with disabled children are exempt from this draconian legislation.
Recent weeks have seen a plethora of personal cases revealed in the media and on the internet. With each revelation the full cruelty of the government's policy becomes clearer.
Couples who sleep in separate bedrooms because of disability or illness have been told to pay up or get out.
A disabled person who has had a chairlift installed in one bedroom has received the same message.
Parents with access to children living with their former partner have learned that keeping a room to enable their children to stay over will face the same financial demand.
Labour chief whip Lord Bassam, who has conducted a survey of the situation in south-east English councils, suggests that the government estimate of 660,000 households facing the your-money-or-your-home ultimatum may be a vast underestimate of the real total, which could be over 900,000.
His lordship, who started political life four decades ago as plain Steve Bassam, a housing campaigner and squatter of empty homes, may well be closer to the mark.
Most of the 900,000 families concerned, 30 per cent of whom are in low-paid jobs qualifying them for welfare benefits, will not choose to move from workplaces or from neighbourhoods in which they have settled and where their children attend school.
They will try to absorb the benefit cut by reducing expenditure on other necessities or will fall into rent arrears, opening the way to eviction.
Those who do decide to move out will find that there are few if any council properties to meet their assessed needs, forcing them into the arms of the more expensive private rented sector.
This will tend to push up the level of housing benefit claimed but, more importantly for the Tories, will create a bonanza for private landlords.
Duncan Smith's brainchild has all the hallmarks of a probable economic cock-up born out of splenetic political dogma, but, unless reversed, it also spells heartache and hardship for countless poor people.
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