In the Times, Tim Luckhurst compares him - unfavourably - to Leni Riefenstahl. His new film is a "poisonously anti-British corruption of the history of the war of Irish independence ... The Wind That Shakes the Barley is not just wrong. It infantilises its subject matter and reawakens ancient feuds." I checked with the production company. The film has not yet been released. They can find no record that Luckhurst has attended a screening - and last night he refused to discuss the matter.

At least Simon Heffer, writing in the Telegraph, admits he doesn't know what he's talking about. Loach, he says, "hates this country, yet leeches off it, using public funds to make his repulsive films. And no, I haven't seen it, any more than I need to read Mein Kampf to know what a louse Hitler was." The Sun says it's "a brutally anti-British film ... designed to drag the reputation of our nation through the mud". Ruth Dudley Edwards in the Daily Mail pronounced it "old-fashioned propaganda" and "a melange of half-truths". She hasn't seen the film either. Nor, it seems, has Michael Gove, who told his readers in the Times that it helps to "legitimise the actions of gangsters".

Are these people claiming that events of the kind Loach portrays did not happen? Reprisals by members of the Royal Irish Constabulary and the Auxiliary division are documented by historians of all political stripes. During the period the film covers (1920-21), policemen visited homes in places such as Thurles, Cork, Upperchurch and Galway and shot or bayoneted their unarmed inhabitants. Nor does any historian deny that they fired into crowds or threw grenades or beat people up in the streets or set fire to homes and businesses in Dublin, Cork, Limerick, Bantry, Kilmallock, Balbriggan, Miltown Malbay, Lahinch, Ennistymon, Trim and other towns. Nor can the fact that the constabulary tortured and killed some of its prisoners be seriously disputed.

It is also clear that some of these attacks were sanctioned by senior officers and politicians. In June 1920, in the presence of the commander of the Royal Irish Constabulary, the force's divisional commissioner in Munster (Colonel GB Smyth) told his men: "You may make mistakes occasionally and innocent people may be shot but that cannot be helped ... The more you shoot, the better I will like you, and I assure you no policeman will get in trouble for shooting any man." He advised that "when civilians are seen approaching, shout "Hands up!" Should the order be not immediately obeyed, shoot and shoot with effect. If the persons approaching carry their hands in their pockets, or are in any way suspicious looking, shoot them down." Sir Henry Wilson, the director of operations in the War Office, complained that he had warned his minister - Winston Churchill - that "indiscriminate reprisals will play the devil in Ireland, but he won't listen or agree". There was even a policy of "official reprisals": the homes of people who lived close to the scene of an ambush and had failed to warn the authorities could be legally destroyed.

Loach's hero, Damien, as many Irishmen were, is radicalised by a raid by the Black and Tans, who were members of the constabulary recruited from outside Ireland. As the film shows, they were responsible for much of the police brutality. The historian Robert Kee, who is a fierce critic of the IRA, remarks that while the police were at first slow to retaliate, their vengeance - exercised against innocent people - "further consolidated national feeling in Ireland. It made the Irish people feel more and more in sympathy with fighting men of their own." The fighter Edward MacLysaght recorded that "what probably drove a peacefully inclined man like myself into rebellion was the British attitude towards us: the assumption that the whole lot of us were a pack of murdering corner boys".

There is no question that the IRA also killed ruthlessly - not just police and soldiers but also people they deemed to be informers and collaborators. But Loach shows this too. (I have seen the film.) The press hates him because he admits that the people who committed these acts were not evil automata, but human beings capable of grief, anger, love and pity. So too, of course, were the British forces, whose humanity is always emphasised by the newspapers. Ken's crime is to have told the other side of the story.

The other side - whether it concerns Ireland, India, Kenya or Malaya - is always inadmissable. The torture and killing of the colonised is ignored or excused, while their violent responses to occupation are never forgotten. The only aggressors permitted to exist are those who fight back.

Does it matter what people say about a conflict that took place 85 years ago? It does. For the same one-sided story is being told about the occupation of Iraq. The execution of 24 civilians in Haditha allegedly carried out by US marines in November is being discussed as a disgraceful anomaly: the work of a few "bad apples" or "rogue elements". Donald Rumsfeld claims "we know that 99.9% of our forces conduct themselves in an exemplary manner", and most of the press seems to agree. But if it chose to look, it would find evidence of scores of such massacres.

In March Jody Casey, a US veteran of the war in Iraq, told Newsnight that when insurgents have let off a bomb, "you just zap any farmer that is close to you ... when we first got down there, you could basically kill whoever you wanted, it was that easy". On Sunday another veteran told the Observer that cold-blooded killings by US forces "are widespread. This is the norm. These are not the exceptions." There is powerful evidence to suggest that US soldiers tied up and executed 11 people - again including small children - in Ishaqi in March. Iraqi officers say that US troops executed two women and a mentally handicapped man in a house in Samarra last month. In 2004, US forces are alleged to have bombed a wedding party at Makr al-Deeb and then shot the survivors, killing 42 people. No one has any idea what happened in Falluja, as the destruction of the city and its remaining inhabitants was so thorough.

Even the Iraqi prime minister, who depends on coalition troops for his protection, complained last week that their attacks on civilians are a "regular occurrence ... They crush them with their vehicles and kill them just on suspicion." But like the Black and Tans the US troops have little fear of investigation or punishment.

Why should we be surprised by these events? This is what happens when one country occupies another. When troops are far away from home, exercising power over people that they don't understand, knowing that the population harbours those who would kill them if they could, their anger and fear and frustration turns into a hatred of all "micks" or "gooks" or "hajjis". Occupations brutalise both the occupiers and the occupied. It is our refusal to learn that lesson which allows new colonial adventures to take place. If we knew more about Ireland, the invasion of Iraq might never have happened.

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