Many observers regarded the Spanish civil war as a dress rehearsal for the second world war. General Francisco Franco's coup against Spain's legally elected government in 1936 seemed to be part of a global struggle between fascism and democracy. Foreign correspondents who covered the Spanish conflict thus became deeply involved in what both sides called “a crusade”. As one newspaperman wrote, they were not so much witnesses as participants in “the horror, tragedy and adventure which constitutes war”.

Nothing inspired those reporting from the loyalist republican side more than the heroic resistance of Madrid. In the words of that outstanding American journalist Herbert Matthews: “The drama, the thrills, the electrical optimism, the fighting spirit, the patient courage of these mad and wonderful people - these are things worth living for and seeing with one's own eyes.” Even the jaded Beaverbrook hack Sefton Delmer, who preferred right-wing rebels to red-tainted republicans, was “swept along in the exhilaration of Madrid's refusal to abandon the fight”.

Delmer and his colleagues were themselves in the front line. Many of them lived in the Florida hotel, which was shelled so regularly they found it hard to sleep. Instead, they conducted a kind of running fiesta, replete with brandy, flamenco, brawls and prostitutes - “whores de combat”, as Ernest Hemingway dubbed them. If the siege stimulated sexual appetites, food was in short supply and journalists had little to eat except chick peas and dried cod. Facing the common danger, however, sharpened their sense of identity with the common people. Arthur Koestler wrote: “Anyone who has lived through the hell of Madrid with his eyes, his nerves, his heart, his stomach - and then pretends to be objective, is a liar.”

Some journalists made no attempt to record the facts impartially, as if to confirm George Orwell's assertion that history stopped in 1936, after which there was only propaganda. Sacrificing the ethics of their profession on the altar of their cause, they told useful lies and suppressed harmful truths. The communist Claud Cockburn, for example, famously invented a story about an uprising against Franco in Spanish Morocco in order to persuade the French government to open the frontier and let the republic receive a vital arms delivery. The ploy worked.

As Paul Preston shows in this excellent study, Mikhail Koltsov, who acted as Stalin's eyes and ears in Spain as well as Pravda's correspondent, was equally unscrupulous. Brilliant, witty and insolent, he once berated Louis Fischer, who contributed well-informed articles to The Nation, for accurately reporting the demoralisation of the republican militia. Koltsov snarled, “You've done more harm than 30 British MPs working for Franco.” Claiming that the Spanish government could have prevented the military insurrection by shooting treacherous generals and other “enemies of the people”, Koltsov justified Stalin's purges in Russia - of which, ironically, he himself became a victim.

Hemingway also accepted the need for communist terror and journalistic deception in the interests of defeating Franco. His friend John Dos Passos protested, “What's the point of fighting a war for civil liberties, if you destroy civil liberties in the process?” Hemingway replied angrily, “Civil liberties shit. Are you with us or are you against us?” Not until after the war did he give a candid account of it, in his novel For Whom the Bell Tolls. There he wrote: “If a thing was right fundamentally the lying was not supposed to matter...but it was a very corrupting business.”

It was especially corrupting on the rebel, nationalist side. Here reporters were harassed and threatened if they showed the least sign of independence. Most, like the mendacious New York Times correspondent William P Carney, were fervent partisans of Franco, delighting in tales of Red bloodshed, looted churches and raped nuns. Censorship was brutally enforced by Captain Luis Bolin, who shocked Noel Monks of the Daily Express by spitting on the corpses of freshly executed “Reds” and saying, “Vermin.”

Bolin was responsible for the biggest lie of the war. He declared that the ancient Basque capital of Guernica had not been bombed by aircraft of Germany's Condor Legion but dynamited by the local people. The truth was told by George Steer, one of the first to witness the devastation, in a despatch to The Times. Penned with dispassionate intensity, it echoed around the world and caused widespread revulsion against fascism.

Steer was by no means alone during the war, as Preston points out, in being a reporter of fierce integrity. Yet such was his devotion to the Basques that he offered to perjure himself on their behalf. Orwell himself gave a distorted picture of the strife out of regard for the Trotskyite faction he had joined. Like many who wrote about it, he succumbed to doublethink, holding contradictory convictions and tampering with reality.

Preston does not do much to explore this schizophrenic state and his prose is clumsy and repetitious. But his book is a splendid monument to scholarship. It is always absorbing, frequently moving and sometimes funny. And it fills a crucial gap in the historio-graphy of the Spanish civil war.

We Saw Spain Die by Paul Preston
Constable £20 pp436

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