Orwell served as both a private and a corporal in Catalonia and Aragon from December 1936 until June 1937. The political party whose militia he served with (POUM, the Workers' Party of Marxist Unification, an anti-Stalinist communist party) was declared an illegal organization and Orwell was subsequently forced to flee or face imprisonment. Having arrived in Barcelona about 26 December he told John McNair, the [[Independent Labour Party]'s representative there, that he had " come to Spain to join the militia to fight against Fascism." He also told McNair that " he would like to write about the situation and endeavour to stir working class opinion in Britain and France." McNair took him to the POUM barracks, where Orwell immediately enlisted. "Orwell did not know that two months before he arrived in Spain, the NKVD's resident in Spain, Aleksandr Orlov, had assured NKVD Headquarters, 'the Trotskyist organisation POUM can easily be liquidated' - by those, the Communists, whom Orwell took to be allies in the fight against Franco."

By his own admission, Orwell joined the POUM rather than the Communist-run International Brigades by chance—but his experiences, in particular his and his wife's narrow escape from the Communist purges in Barcelona in June 1937, greatly increased his sympathy for POUM and, while not challenging his moral and political adhesion to the cause of Socialism, made him a lifelong anti-Stalinist.

On 17 May 1937, Largo Caballero resigned. Juan Negrin, the next prime minister, left the "NKVD-controlled secret police unhindered in its persecution of persons who opposed the Moscow line. On 16 June, when the POUM was declared illegal, the Communists turned its headquarters in Barcelona into a prison for 'Trotskyists'...leaders were handed over to NKVD operatives and taken to a secret prison in Madrid..Nin taken to Alcala de Henares, where he was interrogated from 18 to 21 June..he was then moved to a summer house outside the city which belonged to the wife of Hidalgo de Cisneros and tortured to death..Diego Abad de Santillan remarked; "Whether Juan Negrin won with his communist cohorts, or Franco won with his Italians and Germans, the results would be the same for us."

At the front, Orwell was shot through the throat by a sniper on 20 May 1937 and was nearly killed. He wrote in Homage to Catalonia that people frequently told him a man who is hit through the neck and survives it is the luckiest creature alive, but that he personally thought "it would be even luckier not to be hit at all." After dressing at a first aid post about half a mile from the actual line, he was transferred to Barbastro and then to Lérida where he received only an external treatment of his wound. On the 27th he was transferred to Tarragona and on the 29th May to Barcelona. On 23 June 1937 Orwell and his wife Eileen, with John McNair and Stafford Cottman, boarded the morning train from Barcelona to Paris. They safely crossed into France. Sir Richard Rees later wrote that the strain of her experience in Barcelona showed clearly on Eileen's face : " In Eileen Blair I had seen for the first time the symptoms of a human being living under a political terror."

George Orwell and Eileen returned to England. After nine months of animal husbandry and writing up Homage to Catalonia at their cottage at Wallington, Hertfordshire, Orwell's health declined and he had to spend several months at a sanatorium in Aylesford, Kent.

Because of the book's criticism of the Communists in Spain, it was rejected by Gollancz, who had previously published all Orwell's books: "Gollancz is of course part of the Communism-racket" , Orwell wrote to Rayner Heppenstall in July 1937. Orwell finally found a sympathetic publisher in Frederic Warburg. Warburg was willing to publish books by the dissident left, that is, by socialists hostile to Stalinism.

The book was finally published in April 1938 but "made virtually no impact whatsoever and by the outbreak of war with Germany had sold only 900 copies."

According to John Newsinger, "the Communist vendetta against the book" was ongoing as recently as 1984, when Lawrence and Wishart published Inside the Myth, a collection of essays "bringing together a variety of standpoints hostile to Orwell in an obvious attempt to do as much damage to his reputation as possible."

It should be noted that the following summary is based on a later edition of the book which contains some amendments that Orwell requested: two chapters (formerly chapters five and eleven) describing the politics of the time were moved to appendices. Orwell felt that these chapters should be moved so that readers could ignore them if they wished; the chapters, which became appendices, were journalistic accounts of the political situation in Spain, and Orwell felt these were out of place in the midst of the narrative.

The book begins in late December 1936. Orwell describes the atmosphere in Barcelona as it appears to him at this time. " The anarchists were still in virtual control of Catalonia and the revolution was still in full swing..It was the first time that I had ever been in a town where the working class was in the saddle..every wall was scrawled with the hammer and sickle.. every shop and café had an inscription saying that it had been collectivized." "The Anarchists" (referring to the Spanish CNT and FAI) were "in control", tipping was prohibited by workers themselves, and servile forms of speech, such as "Señor" or "Don", were abandoned. He goes on to describe the scene at the Lenin Barracks (formerly the Lepanto Barracks) where militiamen were given "what was comically called 'instruction'" in preparation for fighting at the front.