The story is one of the most written about events in modern French history. The first large Allied military force to reach Paris on 24 August 1944 was the only French unit in France at the time.

The second armoured division led by General Philippe Leclerc swept aside all opposition - including American objections - to be the first to liberate the capital. What was not known, until now, is that Leclerc's division was hand-picked for the task five months earlier. It was chosen partly because it was French but, more specifically, because its soldiers were white.

According to a book published in France this month, British and American generals insisted in early 1944 that non-white French colonial troops should be excluded from the liberation of Paris. The revelation, drawn from US and UK military archives, coincides with the success of the Franco-Algerian film, Les Indigènes which tells the almost forgotten story of the north African troops who fought in Italy and southern and eastern France in 1943-44. The movie has just been nominated for an Oscar as best foreign film of 2006.

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A book on D-Day and its aftermath published this month by a distinguished French historian, Olivier Wieviorka, includes much other new material from American and British archives. It reveals, for instance, the depths of the crisis of morale which threatened to incapacitate the Allied armies in Normandy a month after the landings on 6 June 1944. At one point, according to US records found by Professor Wieviorka, one in three "wounded" American soldiers suffered from psychological, rather than physical injuries. British infantry fighting spirit at the time was equally poor.

The stated aim of the book (Histoire du Débarquement en Normandie", Seuil, €24) is to tear away some of the legends of glory and "willing sacrifice" surrounding the D-Day invasion. These legends have perhaps survived longer in France than in Britain or America.

The most startling revelation occupies only two out of the book's 416 pages. It throws new light on one of the most mythologised events in French history: the liberation of Paris. At the start of 1944, Leclerc's armoured division was stationed in Morocco. It was chosen, from all other units in the French army to play a headline role in the liberation of the capital because - in the words of one of the most senior US D-Day generals - it was the "only French division which could be made 100 per cent white".

All other units in the French army at that time were two thirds or more African. They fought in Italy and took part in the secondary invasion of France, on the Mediterranean coast, in August 1944. Their role in the defeat of Nazism was little acknowledged during and after the war. Many of the colonial veterans were denied full French army pensions until the Franco-Algerian film Les Indigènes, directed by Rachid Bouchareb, appeared in September last year.

The book reveals that American and British commanders agreed months before D-Day that, for reasons of propaganda and French national morale, a French division should help to liberate Paris. However, they - and not the French leader, General Charles de Gaulle - insisted that the unit must not include colonial troops. In an interview with The Independent, Professor Wieviorka, said that the motives of American and British commanders may have been more political than racial.

"It was agreed that a French unit should be present for the liberation of Paris because that event would inevitably attract great publicity in France and internationally," he said. "Once that decision was made, it was perhaps important to the Allies, for the same propaganda reasons, that the unit should appear French to the people of France. But this was something that the British and Americans insisted on, not De Gaulle."

Professor Wieviorka says that the episode remains perplexing. US military attitudes might have been influenced by the fact that its army refused to allow black conscripts into combat units. On the other hand, the US military made no objection to fighting alongside French colonial troops in southern France.

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Equally, the British view is somewhat puzzling. Comments made by a very senior British officer in the memos found by Professor Wieviorka appear tinged with racial fears about the presence of French colonial troops in Britain. On the other hand, the British Government made no objection to - and ordinary Britons largely welcomed - the black American troops serving in logistic and menial roles with the D-Day invasion force.

A memo from 28 January 1944, found by Professor Wieviorka in the National Archives in Washington, was signed by General Walter Bedell-Smith, Chief of Staff to the Supreme Allied Commander, General Dwight D Eisenhower. General Bedell-Smith, who was to head the CIA in the 1950s, wrote: "It is highly desirable that the [French] division should be composed of white personnel, which points to the second armoured division, which has only one quarter native troops and is the only French division which could be made 100 per cent white."

The Americans initially wanted the French unit to be infantry, not an armoured division. Leclerc's American-built Sherman tanks were awkward to transport by sea all the way from Morocco to Britain. There were plenty of American and British armoured units available in southern England. Infantry was scarce. General Bedell-Smith suggested a way around this problem. "If sea transport problems make it impossible to send an armoured division, we might find it necessary to create a [French] force, from all arms, composed of white troops and designated as a division."

A British general, Frederick Morgan, the officer who headed the D-Day planning team, made the same argument. In a memo written on 14 January 1944 found in the American archives, he wrote: "I am convinced that it is of the greatest importance that there should be French troops among the first units to enter Paris. The bigger these units are the better."

However, the French troops would have to be based in Britain before the invasion. This worried General Morgan and also General Hastings Ismay, Chief of Staff to Winston Churchill, later the first secretary general of Nato. General Morgan wrote to the Americans: "General Ismay and I have made it clear [to the French] that we would accept only with great reluctance anything but troops from France proper... It is unfortunate that the only white French unit is an armoured division stationed in Morrocco... The other French divisions are only 40 per cent white. I told [the French] that they would get [a place in the invasion force] far more easily if they could produce a division of white infantry."

In the event, the decision was made to send the second armoured division from Morocco, with its Sherman tanks but shorn of its 25 per cent non-white troops. The division landed in France on 1 August 1944 and played a role in the Allied break-out from Normandy. Despite American doubts about the wisdom of attacking the capital immediately, Leclerc sprinted east and, on 24-25 August, relieved the army of resistance fighters and policemen which had begun the liberation of Paris. General Leclerc's force was all white - but it was not all French. It contained many volunteers from Spain and a few from Portugal.

There were 550,000 men in the French army in 1944. They were partly assembled from the Free French forces which had gathered around Charles de Gaulle in Britain from 1940. Many others were recruited - not always voluntarily - in the French African colonies. Of these [leaving aside colonists of French origin], there were 134,000 Algerians, 73,000 Moroccans, 26,000 Tunisians and 92,000 men from colonies in black Africa.

This multiracial army was first thrown into battle in Italy in 1943, particularly in the grim struggle to dislodge the Germans from Monte Cassino. The same troops landed with the Americans in the south of France on 15 August 1944, while the main German force was still engaged in Normandy. After advancing through France with little opposition, the southern invasion force became involved in terrible winter fighting against the German armies which had assembled to defend the approaches to the Reich in the Vosges mountains and in Alsace in north eastern France from December 1944.

Les Indigènes, follows the boot-steps of four north African soldiers, all played by well-known French actors of Arab origin: Jamel Debbouze (Amélie), Samy Naceri (Taxi), Roschdy Zem and Sami Bouajila. Bernard Blancan plays a pied-noir (white Algerian colonist) sergeant, who is revealed - to his fury - to be half-Arab. All five men shared the male actor's prize when the movie was shown at the Cannes Film Festival last May.

Professor Wieviorka's book also includes new material on the collapse of the morale of British and American troops in Normandy a month after the D-Day landings. The reluctance of some Allied soldiers to fight - and the reluctance of senior officers to send them to their deaths - has already been described in classic recent accounts of the battle of Normandy by the British historians Max Hastings and John Keegan.

Professor Wieviorka adds a mass of telling detail. According to US Army medical records, a third of all soldiers listed as "wounded" in mid-July 1944 suffered from psychological, not physical, injuries. Both the British and American armies suffered an epidemic of self-inflicted wounds at this time, the book says. Professor Wieviorka quotes an account by one of Eisenhower's aides which says that the Supreme Allied Commander was "depressed" to find 1,100 cases of self-inflicted wounds by American soldiers when he visited a large field hospital near Carentan in Normandy in July 1944.

At this time, after the initial success of the landings, the British, Canadian and American armies were being held back by the Germans in a series of murderous close-quarter battles comparable to the bloody struggles on the Eastern Front or in the First World War. Morale rose sharply after the Americans broke through on 24 July.

In his book, Professor Wieviorka suggests that part of the problem was that many of the US and British infantry in Normandy were a "weak link" - poorly selected and undertrained. "Thrown into infantry units by default, less educated than the average sailor or airman, they often had little pride in their units," he writes. "And yet the battle fell largely on their shoulders ... the infantry took 76 per cent of all battle losses in Normandy and 76 per cent of the psychiatric cases".

Professor Wieviorka said his intention was not to denigrate the achievements of the Allies. "Quite the opposite," he said. "The standard picture of young men ready and eager to lay down their lives to defeat Nazism is too simplistic. Only when you look at what these troops really went through do you have a proper understanding of their achievements."