This book is Orwell's account of his time fighting on the Republican side of the Spanish Civil War. Orwell was in Spain from late December 1936 until June 1937, fighting as an international volunteer with the POUM militia. His account covers three main subjects: his time fighting on the frontline; his experience of the street fighting in Barcelona between different republican factions in May 1937 and the events that followed; and his analysis of the different republicans parties and the struggles between them leading up to the suppression of POUM.
Orwell spent the first four months of 1937 fighting with POUM militia around Huesca near Saragossa, initially with a mainly Spanish unit, then with one of English volunteers. "Fighting" is perhaps not the best word, for as Orwell himself says, very little actual fighting occurred, and most of the effort on his section of the front was concentrated on trying to keep warm and fed, although he did take part in one sharp but inconclusive assault.
In late April, Orwell returned to Barcelona on leave. While there, at the start of May, the government police attempted to seize control of the Telephone Exchange from the Anarchist unionists who were occupying it. POUM came out on the side of the Anarchists, and street fighting followed by several tense days of barricaded stand-off followed, during which Orwell took part in guarding the POUM headquarters. Government police were brought in from Valencia, and order restored. Soon after the fighting ended, Orwell returned to the frontline; however, the events in Barcelona were to have serious consequences for POUM.
Orwell had been back at the frontline for only ten days when he was hit by a Fascist sniper. The bullet passed through his neck, and came very close to killing him. He was evacuated from the frontline and sent first to a hospital in Tarragona, then to a POUM-run sanatorium on the outskirts of Barcelona. He had lost his voice, it was believed permanently, and his other wounds would take months to fully heal, so he decided to leave Spain, and travelled back to the front to get his medical discharge papers.
By the time he arrived back in Barcelona around the 20th of June, POUM has been suppressed by the republican government, and its members were being rounded up and imprisoned. POUM was blamed for the troubles of early May; but more seriously than that, they were accused by the now-dominant Communists of being Trotskyists and Fascist traitors. After dodging the police for a few days, Orwell managed to slip out of the country.
(Excerpt from: http://www.williamwebber.com/books/homcat.html)