On many occasions films have been changed so that the US armed forces are shown in a more heroic fashion. Film companies agree to the changes because doing so saves them millions in production costs. If film-makers do not agree to alterations, assistance is withheld.

Among films that have been given approval and help by the Pentagon are Armageddon, Air Force One, The Jackal, Pearl Harbour and Top Gun. Those that have failed the test include Forrest Gump, Mars Attacks!, The Thin Red Line, Apocalypse Now, Sgt Bilko, Platoon and Independence Day.

One internal army memo about Forrest Gump, which starred Tom Hanks, suggested that "the generalised impression that the army of the 1960s was staffed by the guileless or by soldiers of limited intelligance" was unacceptable. "This impression is neither accurate nor beneficial to the army."

Of the scene when Tom Hanks shows a scar on his buttock to President Johnson, a navy memo states: "The 'mooning' of a president by a uniformed solider is not acceptable cinematic licence."

The documents indicate that the Pentagon sees the film business as an important part of public relations. "Military depictions have become more of a 'commercial' for us," said one memo quoted in an investigation by David Robb in the current issue of the media magazine Brill's Content.

In GI Jane, the 1997 film starring Demi Moore, one scene in a foxhole originally showed a male serviceman having difficulty relieving himself in her presence. "While addressing issues related to the presence of women in front-line ground combat, the urination scene in the foxhole carries no benefit to the US navy," wrote US navy commander Gary Shrout to the director, Ridley Scott. Scott wrote back that "this scene has been eliminated" and agreed to other changes but the end result was still unacceptable.

In Hearts in Atlantis, due out later this year and starring Anthony Hopkins, there is no military plot but the film-makers wanted to use land belonging to the army. The Pentagon agreed and suggested that the film could include a shot of an army recruiting booth in a carnival scene.

When Top Gun, starring Tom Cruise, opened in the US, navy recruiting booths were set up in cinemas. Cooperation had been given after the character played by Kelly McGillis was changed from an enlisted woman to someone outside the service, as relationships between officers and enlisted personnel are forbidden in the navy.

The film companies are often shown in the documents to be more than anxious to help. "We firmly believe that with the support of the US military, Armageddon will be the biggest film of 1998, while illustrating the expertise, leadership and heroism of the US military," wrote Disney executive Philip Nemy to the Pentagon.

The Jackal, starring Bruce Willis and Richard Gere, received help after the marines were given a better role. Major Nancy LaLuntas had objected that the helicopter pilots had no "integral part in the action - they are effectively taxi drivers."

A letter from film's director, Michael Caton-Jones, stated: "I am certain that we can address the points that you raised ... and effect the appropriate changes in the screenplay that you requested."

In GoldenEye, the 1995 James Bond film, the original script had a US Navy admiral betraying state secrets, but this was changed to make the traitor a member of the French navy - after which cooperation was forthcoming.

The makers of Independence Day agreed to turn the secretary of defence, under whom military installations fell to alien invaders, into the White House chief of staff, but still did not win approval.

The writer and producer, Dean Devlin, had told the Pentagon: "If this doesn't make every boy in the country want to fly a fighter jet, I'll eat this script." But a Department of Defence memo concluded: "The military appears impotent and/or inept; all advances in stopping aliens are the result of actions by civilians."

Mars Attacks! and the comedy Sgt Bilko also failed the test.

The financial incentives for film companies are great because military hardware is enormously expensive and difficult to hire, with the Israeli air force being one of the few services that rents out its equipment.

Philip Strub, special assistant for the entertainment media at the Pentagon, said yesterday that the military was often asked to help when a film was still in development. He said that after changes had been suggested it was a matter of trust that the film-makers would honour the changes and he was not aware of any injunction ever being taken to stop a film being shown: "It would be anathema to us (to interfere with) the artists' rights and first amendment rights ... We regard it as a success when we work with a film-maker on a project and a lack of success when we don't."

Some films the Pentagon had been unable to assist, he said. Saving Private Ryan was shot in Europe where the US had no second world war equipment. Some projects, like the anti-war Born on the Fourth of July, never asked for help, he said. He added he anticipated that growth of computer-generated imagery meant that requests for help would decrease.

Cheryl Rhoden of the Writers Guild of America West, said yesterday that she was aware of the issue. "Any time that any outside entity attempts to effect changes is of concern to writers," she added.

Films which obtained cooperation:

• Air Force One

• The Caine Mutiny

• A Few Good Men

• From Here to Eternity

• Armageddon

• The Longest Day

• The Hunt for Red October

• Pearl Harbour

• Patton

• Patriot Games

• Top Gun

• Windtalkers (to be released)

• The Jackal

• Hamburger Hill

• Hearts in Atlantis (to be released)

• The Longest Day

• GoldenEye

• The American President

• Behind Enemy LInes (to be released)

• Apollo 13

• Tomorrow Never Dies

• Tora! Tora! Tora!

• A Time to Kill

Films denied cooperation:

• Apocalypse Now

• Catch-22

• Broken Arrow

• Die Hard 2

• Dr Strangelove

• Forrest Gump

• Full Metal Jacket

• GI Jane

• Independence Day

• The Last Detail

• Lone Star

• Mars Attacks!

• Memphis Belle

• An Officer and a Gentleman

• Platoon

• Sgt Bilko