Special report: How my father's face turned up in Robert Capa's lost suitcase

The great war photographer was not one person but two. Their pictures of Spain's civil war, lost for decades, tell a heroic tale

In 2007 a long-lost suitcase, forgotten for over half a century in the house of a retired general in Mexico, was found to contain 4,000 photographs of the Spanish Civil War. Many of them were by Robert Capa, the most famous war photographer of the 20th century. The existence of the suitcase had been rumoured since it disappeared in 1940, at the time of the Nazi invasion of France, but the survival of its contents seemed unlikely. In terms of shock and drama, the discovery was to war photography what the finding of the tomb of Tutankhamun was to the study of ancient Egypt.

The thousands of images on 126 ageing rolls of film convey with stark immediacy the violence, suffering and destruction of the war in Spain between 1936 and 1939. But there is far more in these pictures than the melodrama of military combat. They portray the hopes and courage of people struggling against the odds to turn back fascism and military dictatorship in a ferocious conflict that gave a foretaste of what was to come in the Second World War. Photographs show Republican pro-government militiamen inching their way through buildings, shattered by shells and bombs, that look like the ruins of Stalingrad; soldiers advance half-crouched against incoming fire and later stagger back carrying the wounded on their shoulders; a woman nurses a baby during a land reform meeting in Estremadura a few months before the Nationalist military coup in 1936; three years later, militiamen mobilise for the final vain defence of Barcelona; and, finally, utter defeat, with pictures of despairing lines of refugees making their way across the Pyrenees to the grim half-life of internment camps in the south of France.

The negatives that arrived in New York from Mexico six years ago were in fact taken by three different and immensely able photographers: Robert Capa, Gerda Taro and David Seymour (known as "Chim"). Of these, Capa is now by far the best known, his most famous pictures taken in the Spanish Civil War and at Omaha Beach during the D-Day landings. The three photographers were Jewish emigrants from Hungary, Germany and Poland, all more or less on the run from fascism and close to the Communist Party. They were highly politicised people who worked for small left-wing magazines and papers until the quality of their work attracted the attention of international publications.

The situation was more confusing than it appears, because Robert Capa did not exist except as a byline and was the collective nom de plume of two of the photographers: Andre Friedmann and Gerda Taro. Impoverished and ill-paid immigrants in Paris, they had the ingenious idea of offering photo agencies pictures from the supposedly famed American photographer Robert Capa, whom they had just invented. Taro (whose real name was Gerta Pohorylle and had been born in Stuttgart) and Friedmann, who came from Hungary, claimed to be Capa's sales representatives, but the excellent pictures were their own. American photographers had a high reputation in France, and Friedmann and Taro were able get three times more money for photographs by the mythical Capa than they would if they had operated under their own names.

The trick was almost too successful. After Taro was killed in the Battle of Brunete, west of Madrid, on 26 July 1937, her name was largely forgotten. Friedmann became so identified with Capa that people had come to assume it was his real name by the time he died stepping on a landmine in Indo-China in 1954. He famously advised photographers that "if your pictures aren't good enough, then you're not close enough". I am not entirely sure this is correct, but Capa, Taro and Chim (despite his English name he was a Polish Jew) were all killed in action. Two years after Capa's death, Chim was fatally wounded by Egyptian gunfire during the 1956 Suez crisis.

I only heard of the Mexican suitcase last year when a friend told me that he had seen a photograph of my father, Claud Cockburn, in an exhibition of the photos in Paris. I looked up the pictures and immediately found one of him, looking older than his 33 years, wearing a white shirt, round glasses and with his dark hair unruly and receding. Beside him is Fred Copeman, a former sailor in the Royal Navy who was briefly commander of the British battalion in the International Brigade. There is scrubland behind the two men and the caption says the picture was taken by Gerda Taro at Brunete in July 1937, just a few days before she was killed. There is nothing in the picture to show that Brunete, 15 miles west of Madrid, was one of the most savage battles of the civil war, in which the Republicans lost some 20,000 and the pro-Franco Nationalists 17,000 dead and wounded.

My father, who had joined the Communist Party in 1933, was in Spain as correspondent for The Week, his own newsletter which was a sort of early version of Private Eye, and The Daily Worker. By chance, he had been in Spain when General Francisco Franco and other Spanish generals launched their half-successful coup against the elected Republican government on 17 July 1936. (Having visited Spain only once before, Claud was unable to persuade people that his arrival there just as the Spanish army launched its putsch was neither amazing political foresight nor part of a fiendish Comintern plot.) He joined the Republican militia soon after and, though he had had only brief military training at school, commanded a platoon of heroic but untrained peasants. He later recorded his dismay as they charged Franco's Moorish gunners "holding rifles high above their heads with one hand and giving the clenched fist salute with the other. When they saw me dodging along, bent half-double and taking whatever cover there was, they thought the posture unworthy, despicable." Many were killed or wounded before he could persuade them that propaganda posters on the walls of Madrid should not be taken as tactical advice on how to storm an enemy position.

There is no picture of Taro and my father together but he was obviously attracted to the young photographer with her strawberry-blond hair, charm, good looks, bravery and political sophistication. By one account, to be near her he moved into the Casa de Alianz in Madrid, an expropriated villa often used by journalists, where she was staying. She was a self-confident 26-year-old briefly jailed in Germany for distributing anti-Nazi literature before fleeing to France on a fake passport. She and Friedmann were lovers but their relations may have been strained by the summer of 1937 when he went back to Paris to sell their photographs. He never saw her again.

In July Taro and my father went together to cover the Brunete offensive by the government forces, which was intended to relieve Nationalist military pressure on Madrid and Bilbao and persuade the rest of the world that Franco's victory was not inevitable. After his experiences in the anti-Franco militia, Claud found that the "makeshift, ramshackle quality of the Spanish War could be terrifying because it kept reminding me of the odds against our sort of forces being victorious over the trained troops of the other side." This is what happened at Brunete where Franco's Nationalists launched a devastating counter-offensive backed by the air power of Germany's Condor Legion. Claud and Taro reached the front on 22 July and were promptly strafed in a field by German aircraft. Capa's biographer Richard Whelan cites Claud saying that he and Taro came "around to the calculation that we had, this time, very little chance of getting out alive. She then stood up and began to take photographs of the planes, saying 'in case we do get out of this, we'll have something to show the Non-Intervention Committee'." The committee, on which many countries were represented, was meant to prevent both sides receiving military aid, but ignored massive German and Italian support for the Nationalists.

Taro and my father escaped machine-gun fire from the German aircraft, but he later recalled her saying that "when you think of all the fine people we both know who have been killed even in this one offensive, you get the feeling that it is somehow unfair to be alive". Later they went back to the Alianz in Madrid and she was planning to return to Paris on 26 July to meet up withFriedmann. But at the last moment she decided on one more visit to the front, which by this stage was caving in under the weight of the Nationalist assaults. She went with a friend called Ted Allan, a young Canadian attached to a medical unit, but conditions had got so dangerous that their driver abandoned them short of Brunete and they had to walk the last part of the journey. They went to see the Republican commander, but he ordered them and other journalists to leave immediately because Franco's forces were about to attack. Ignoring his advice, they stayed, but the Republican troops began to panic and retreat pell-mell back down the road to Madrid. Taro jumped on to the running board of a car carrying wounded soldiers, but an out-of-control Republican tank accidentally side-swiped the vehicle, mortally wounding Taro, whose stomach was ripped open. She died in a hospital at El Escorial, her last words being, according to an American nurse: "Are my cameras smashed? They're new. Are they there all right?"

Her funeral in Paris was attended by tens of thousands and she was seen as a martyr for democracy and anti-fascist forces. The sculptor Alberto Giacometti was commissioned to design a memorial. But within two years the memories of the Spanish war were submerged by the even more awful carnage of the Second World War. It was forgotten that Capa had originally been two people. American magazines, when they referred to Taro at all, said she had been Capa's wife. In fact, he had suggested marriage shortly before she was killed but she had refused. It was only with the discovery of the contents of the Mexican suitcase, one third of the pictures being hers, that her reputation began to revive.

How did the suitcase end up in the closet of a retired general in Mexico City? In October 1939 Robert Capa, as Friedmann was now known, sailed for New York, a wise move as a Jew who was a militant anti-fascist and close to the Communists. He left all the negatives in his studio in the care of his dark-room manager and fellow photographer, Imre "Csiki" Weiss. "When the Germans approached Paris," wrote Weiss, another Hungarian Jew, "I put all Bob's negatives in a rucksack and bicycled it to Bordeaux to try to get it on a ship to Mexico. I met a Chilean in the street and asked him to take my film packages to his consulate for safe keeping. He agreed." Weiss was interned in French Morocco but survived.

Other accounts of what happened differ, but whatever its exact itinerary the suitcase ended up in the possession of General Francisco Aguilar Gonzalez, the Mexican ambassador to Vichy in 1941-42. Mexico was one of the few countries to give visas to refugees from fascism, which may explain how Aguilar ended up in possession of the suitcase. It is not known who gave it to him and he may not have known what was in it or, if he did, did not consider the rolls of film important. The General died in Mexico City in 1971 and his effects went to a friend, who was the aunt of the Mexican film-maker Benjamin Tarver. When she died, he inherited the case and, after seeing an exhibition about the Spanish Civil War, he sought advice on what to do with the negatives. Word began to filter out in 1995 that they had survived, though, even then, it was another 12 years before they reached the International Center of Photography in New York, where they were fully examined and their worth recognised.

My father kept moving in and out of Spain during the rest of the war and ultimately escaped across the Pyrenees to France before the final collapse. He always believed that the odds were heavily stacked against the government, because Franco's forces had trained officers, cohesive command as well as the strong support of Hitler and Mussolini. The courage and commitment of the Republicans, even when backed by Soviet arms and advisers, were never going to counter-balance the fascist alliance. At the same time, he went on fighting very hard to avert defeat as a journalist and propagandist for the Republic, commenting later that intellectuals often become overly disillusioned or dispirited by mistakenly exaggerating the finality of any particular defeat or victory. Others, more heroically, proved in Spain in the 1930s – and here he might have been thinking of Taro – that "when some people talk about dying for a cause they mean it".

The contents of the Mexican suitcase are in the hands of the International Center for Photography, in New York