TONY BLAIR is intent on following George Bush into a devastating and murderous war against Iraq. Fury at this has produced the biggest anti-war movement we've ever seen in Britain. Opinion polls last week showed a mere 13 percent of people wholeheartedly supporting Bush and Blair. Even the BBC now talks about "majority opposition to the war". Yet war still seems probable. What can we do about this? We cannot rely on normal parliamentary methods.

Blair refuses to allow parliament to vote on war in case it shows how unenthusiastic his own party is for war. He hopes that once war has started he can use media hysteria about "stabbing our boys in the back" to force most of the parliamentary doubters into line. "Parliamentary democracy" cannot stop Blair ignoring majority opinion.

The only way we can impede the drive towards war is to look to extra-parliamentary action and the power of a mass movement. War is the hardest test governments face short of all-out revolution. Wars are unpredictable. Governments do not know in advance how long wars will last or the degree to which they will produce bitter conflicts within their own societies.

Rulers across Europe expected the war that began in August 1914 to be over by Christmas. It dragged on for four years and led to the collapse of three of the empires that started it. The US government first sent troops to Vietnam in the early 1960s believing they would easily subdue the local population. When things go wrong in war, bitter social struggle can erupt at home, and discipline can begin to collapse within the armed forces themselves. This happened during the First World War and again in the Vietnam War.

The bigger the open and organised opposition to the war, the more rapidly such social struggles can erupt and the more rapidly morale in the armed forces can collapse, turning foreign war into an uncontrollable domestic crisis. This is why governments are always eager to build "consensus" for their wars.

Collaboration in the war effort by all three mainstream parties and the trade union leaders was vital to British governments in both world wars. After the 1982 Falklands War, Tory leaders made it clear that waging the war had depended on the support of the Labour opposition led by Michael Foot.

By contrast a Tory government abandoned its 1956 Suez war and ditched the prime minister, Anthony Eden. Part of the reason for that was the opposition of half the population, including the Labour Party and papers like the Daily Mirror and the Observer. Blair knows he faces even bigger opposition today, despite the unity of the parliamentary front benches.

When the first big demonstration took place against the war on Afghanistan in October 2001 the media and the government ignored it. Government leaks showed that Blair was shocked by the size of the second demonstration two months later. The huge 400,000-strong demonstration in September last year thrust its way into the media and pushed extra-parliamentary opposition to the war to the centre of the political agenda.

We know this was part of the reason George Bush spent months getting his resolution through the Security Council in the autumn. His advisers warned there would be only limited support from US voters if he went to war without at least one major foreign ally, Britain.

Tony Blair told Bush that it would be easier to provide such support if he could line up other UN Security Council members. There are reports that Blair has been urging further delay on Bush and, preferably, another UN Security Council motion, in an effort to weaken opposition to the war here.

In other words, the anti-war movement has already had a significant effect. Demonstrating has made a difference. What matters is not just what happens here and in the US. A succession of field marshals, generals and former ambassadors have been warning Blair that there could be bitter fighting for control of Baghdad and that this could provoke upheavals against US-backed dictatorships elsewhere in the oil-rich Middle East.

People like US defence secretary Donald Rumsfeld dismiss such forecasts, claiming that Saddam will fall at the first blow. It is impossible to tell in advance who is right. But it is those with most knowledge of the Middle East in the British Foreign Office and the US State Department who are the most worried. They know the Middle East's dictatorships will be safe so long as it is merely a matter of small, secret groups planting bombs against US and Israeli targets.

These are annoying, but at the end of the day the imperialist secret services can cope. What they really fear is what happened to the US's biggest regional client in the 1970s, the Shah of Iran. A growing wave of massive demonstrations spurred the oil workers to strike and split the army, forcing the Shah to flee the country.

The globalisation of telecommunications means that today protests in one part of the world spur protests elsewhere. The mass demonstrations in Britain have been front page news across the world, contributing to the build-up of an international movement which expects to bring ten million people onto the streets on 15 February.

Simply to demonstrate opposition to war and then go home and sit back while Bush unleashes his bombers and his troops with Blair's connivance is not enough. To stop war we need to go further, to radicalise the opposition and build more militant action. The discussion taking place within the anti-war movement about direct action is a recognition of this.

But there are different emphases about what kind of direct action people should look to. There is a tradition in some left wing and peace circles of seeing direct action as something either cut off from or counterposed to mass action.

The emphasis is on small groups of people breaking into military bases remote from centres of population or undergoing special training in non-violent direct action. There have even been cases of people going further when such methods do not work, and taking to violent actions in small groups. This happened at one stage in the US movement against the Vietnam War with the formation of the Weather Underground, which planted bombs in military-related institutions.

Such small group actions cannot stop our rulers. They are like gnat bites on the war machine. We do need militant, direct action. And such actions will at first involve minorities. But to be effective such actions need to aim to connect with the wider movement, and to encourage the biggest numbers of people that they too can take similar action. This is because what really terrifies the warmongers are huge movements that shake the whole of the society on which the war machine's functioning depends.

What shook them during the First World War was the influence anti-war ideas eventually had on striking munitions workers and mutinying soldiers. The movement against the Vietnam War had a real impact when mass demonstrations helped its message feed into the struggle for black liberation in 1968, produce occupations across hundreds of universities in 1970, and then infect the US army in Vietnam in 1971 and 1972. The anti-war movement today is hundreds of times bigger than at the beginning of these wars.

We have to draw the widest number of people into it, and create the basis for massive numbers of people taking direct action in the places where they work, study and live. The Scottish train drivers who refused to move munitions recently gave a small but very significant sign of the kind of direct action that is possible. So did those students who occupied their colleges on the day of action against the war on 31 October. Such actions are not always easy. They involve arguing with workmates and neighbours, as well as clashes with the powers that be.

But when they are successful, and when they spread to involve more and more people, they shake those powers in a way in which minority actions cut off from the mass of people never do. Work in this direction has to begin in the three-week build-up to 15 February.

We need to move on from big all-city rallies against the war to the building of groups of anti-war activists in every workplace, in every faculty of every university and every site of every sixth form centre and FE college, in every locality.

Everywhere we have to carry the argument that Blair has no mandate to bomb and kill in our name and we are going to do our utmost to stop him.

Everything we do has to be directed to this end. This does not mean activists waiting for everyone else to move. It does mean measuring what we do by our ability to draw others behind us. The British police hinted in recent discussions about the organisation of anti-war protests that what really worried them was not old-style "civil disobedience" but something much more serious, "civil unrest".

This is the ultimate fear of any government heading for war. Late in 1966, US president Lyndon Johnson's generals told him of computer calculations showing the impact of using the same methods against the North Vietnamese capital, Hanoi, as had been used against Hiroshima and Nagasaki in 1945. Johnson replied, "I have one question to ask your computers-will you feed into it how long it will take 500,000 angry Americans to climb the White House wall out there and lynch their president if he does something like that?"

We have to ensure that Tony Blair has similar nightmares about the impact of war on Iraq. The next step in doing so is in turning the whole of central London into one massive anti-war demonstration on 15 February. The bigger that demonstration the greater the potential to build beyond it to the mass, militant action that can challenge and stop the war.

A huge angry march can become a springboard to the mass "civil unrest" that the authorities fear.