It is in the nature of protest marches that they tend to be a reaction to decisions that have already been taken and are most unlikely to be reversed. Thus it was on the Day of Action in 1980, the People's March for Jobs in 1981 and 1983, the CND march against Polaris in October 1983 and, yes, even though the House of Commons had yet to go through the formal motions, the march against the Iraq war in February 2003.
The one instance often cited as having led to a change of policy, the poll tax demo of 1990, which slid into violence, was not the cause of the abolition of the community charge. That happened because Conservative MPs realised that they were heading for election defeat, and that they could change the policy only if they changed their leader.
So it is with yesterday's March for the Alternative. This newspaper shares the broad sentiment behind the march – while, of course, deploring the violence, and recognising that its placards act as an umbrella for a wide range of disparate and possibly incompatible views. The Independent on Sunday believes that the spending cuts are going too far and too fast, and was unconvinced by the reasons Nick Clegg and Vince Cable dreamt up to justify their declaration that black was white last May. There was an alternative to the coalition's policy, similar to that set out by Gordon Brown and Alistair Darling last year, and which is being pursued successfully by the Obama administration in the United States now.
However, that issue has now been settled, unfortunately. There may be room for adjustment around the edges, but the basic plan has been decided and the alignment of forces behind it in the Commons is unlikely to shift over the next four years. As John Rentoul argues today, by the next election, the positions that the parties take now on the deficit will have been superseded by new arguments about the next five years.
So does that mean the hundreds of thousands who took part in the march were wasting their time? Absolutely not. What is important, and what will still matter by the time of the election, is that the Government has failed disastrously to grasp the principle of fairness.
Whatever the wisdom of addressing the march at all, and whatever the judgement of comparing it with the struggles of the suffragettes, the civil rights movement and the campaign against apartheid, Ed Miliband was right to ask: "Where is the fairness?" And he was right to point out that ministers, and especially the Chancellor of the Exchequer, keep saying, "We are all in this together," when patently we are not.
This is a blind spot shared by David Cameron and George Osborne. For all the Prime Minister's much-vaunted feel for politics, he and Mr Osborne come across as having no understanding at all of what life is like for people worrying about making their money last until pay day. This is not just about the low paid or people on benefits, but about several of the tribes of the middle class that we described last week. Michael Gove, the Secretary of State for Education, tells children which books they should read while his government's policies are closing libraries all over the country.
Whatever the pace and depth of the cuts, what will still matter at the next election is whether or not they are seen as fair. In that, yesterday's march spoke with what Mr Miliband rightly called "the voice of the mainstream majority". What is surprising about Mr Cameron, Mr Osborne and Mr Clegg is how far short they have fallen in understanding the anger felt by the majority that, when it comes to fixing the deficit, the wealthiest have got off so lightly. The idea that raising £2.5m from the banks, plus last week's £2bn from North Sea oil companies and £1bn from measures against tax avoidance is enough to blunt that anger is naive in the extreme. This is not a matter of mere rhetoric but of substance.
It is on this ground that the Opposition should prepare to fight. As the Labour Party's policy reviews start the work of setting out "the alternative" in the name of which the people marched yesterday, they need to move on from the old argument about the speed of the cuts and on to the territory of fairness. We all accept that there has to be some financial pain. But there will be a heavy price to pay at the next election if it is not shared fairly and seen to be so.
Yesterday's march was a warning that the coalition parties will pay the ultimate price that can be exacted in a democracy if they do not ensure that the burden is more equitably borne.