In the early hours, two days before the attack on Iraq began, two men in their 30s, Phil Pritchard and Toby Olditch, cut through the fence surrounding the air base at Fairford in Gloucestershire and made their way towards the B52 bombers which were stationed there. The planes belonged to the US air force. The trespassers were caught by guards and found to be carrying tools and paint. They confessed that they were seeking to disable the planes, in order to prevent war crimes from being committed. This year they were tried on charges of conspiracy to commit criminal damage, which carries a maximum sentence of 10 years. Last week, after long deliberations, the jury failed to reach a verdict.

The same thing happened a month ago. Two other activists, Margaret Jones and Paul Milling, had entered the same RAF base and smashed up more than 20 of the vehicles used to load bombs on to the B52s. The charges were the same, and again the jury failed to agree. In both cases the defendants claimed to be putting the state on trial. If I were in government, I would be starting to feel uneasy.

The defendants had tried to argue in court that the entire war against Iraq was a crime of aggression. But in March this year the law lords ruled that they could not use this defence: while aggression by the state is a crime under international law, it is not a crime under domestic law. But they were allowed to show that they were seeking to prevent specific war crimes from being committed - principally, the release by the B52s of cluster bombs and munitions tipped with depleted uranium.

They cited section 5 of the 1971 Criminal Damage Act, which provides lawful excuse for damaging property if that action prevents property belonging to other people from being damaged, and section 3 of the 1967 Criminal Law Act, which states that "a person may use such force as is reasonable in the prevention of a crime". In summing up, the judge told the jurors that using weapons "with an adverse effect on civilian populations which is disproportionate to the need to achieve the military objective" is a war crime. The defendants are likely to be tried again next year.

While these non-verdicts are as far as the defence of lawful excuse for impeding the Iraq war has progressed in the UK, in Ireland and Germany the courts have made decisions - scarcely reported over here - whose implications are momentous. Last year, five peace campaigners were acquitted after using an axe and hammers to cause $2.5m worth of damage to a plane belonging to the US navy. When they attacked it, in February 2003, it had been refuelling at Shannon airport on its way to Kuwait, where it would deliver supplies to be used in the impending war. The jury decided that the five saboteurs were acting lawfully.

This summer, the German federal administrative court threw out the charge of insubordination against a major in the German army. He had refused to obey an order which, he believed, would implicate him in the invasion of Iraq. The judges determined that the UN charter permits a state to go to war in only two circumstances: in self-defence, and when it has been authorised to do so by the UN security council. The states attacking Iraq, they ruled, had no such licence. Resolution 1441, which was used by the British and US governments to justify the invasion, contained no authorisation. The war could be considered an act of aggression.

There is no prospect that the British prime minister could be put on trial for war crimes in this country (although, as the international lawyer Philippe Sands points out, there is a chance that he could be arrested and tried elsewhere). Even so, the government appears to find these legal processes profoundly threatening.

When the Fairford protesters took their request to challenge the legality of the war to the court of appeal, Sir Michael Jay, permanent under-secretary at the Foreign Office, submitted a witness statement which seems to contain a note of official panic.

"It would be prejudicial to the national interest and to the conduct of the government's foreign policy if the English courts were to express opinions on questions of international law concerning the use of force ... which might differ from those expressed by the government," he wrote. Such an opinion "would inevitably weaken the government's hand in its negotiations with other states. Allied states, which have agreed with and supported the United Kingdom's views on the legality of the use of force, could regard such a step as tending to undermine their own position."

It doesn't seem to matter how many journalists, protesters or even lawyers point out that the British government had no legal case for attacking Iraq, that the attorney general's official justification was risible and that Blair's arguments were mendacious. As long as the government has a majority in parliament, the support of much of the press and an army of spin doctors constantly weaving and reweaving its story, it can shrug off these attacks. It can insist, with some success, that we "move on" from Iraq. But an official verdict, handed down by a court, is another matter. If a ruling like that of the German federal administrative court were made over here, it could be devastating for Blair and his ministers.

The prosecutors have lost before. In 1999, a sheriff (a junior Scottish judge) at the court in Greenock instructed the jury to acquit three women who had boarded a Trident submarine testing station on Loch Goil and thrown its computers into the sea. They had argued that the deployment of the nuclear weapons carried by the submarines contravened international law. The sheriff said she could not "conclude definitively" whether or not this was true, but that she had "heard nothing which would make it seem to me that the accused acted with criminal intent". The court of session in Edinburgh later overturned her ruling. Now campaigners against nuclear weapons will be mounting further legal challenges, as they try to sustain a continuous peaceful blockade of the Trident base at Faslane for a year (see www.faslane365.org).

In 1996, four women were acquitted of conspiracy and criminal damage after disabling a Hawk jet which was due to be sold by BAE to the Suharto dictatorship in Indonesia. They argued that they were using reasonable force to prevent crimes of genocide that the Indonesian government was committing in East Timor. Their acquittal might have helped persuade Robin Cook to seek to introduce an "ethical dimension" to foreign policy in 1997 (he was, as we now know, thwarted by Blair).

It is true that such verdicts (or non-verdicts) impose no legal obligations on the government. They do not in themselves demonstrate that its ministers are guilty of war crimes. But every time the prosecution fails to secure a conviction, the state's authority to take decisions which contravene international law is weakened.

These cases cannot reverse the hideous consequences of the crime of aggression (the "supreme international crime", according to the Nuremberg tribunals) that Blair and Bush committed in Iraq. But they do make it harder to repeat.

· George Monbiot's book Heat: How to Stop the Planet Burning is published by Penguin.



www.monbiot.com