George Orwell's life-long sympathies with left and socialist causes naturally led him to align himself with the anti-Fascist Republican forces in the Spanish Civil War. So it was, in late 1936, that he journeyed to Barcelona, eventually joining POUM (Partido Obrero de Unificacion Marxista), or Worker's Party of Marxist Unification, which was led by Andreu Nin, whose Trotskyite politics were both anti-Fascist and anti-Stalinist.
As he later admitted, Orwell joined the POUM faction mostly by chance; John McNair, his contact in Barcelona, took him to their headquarters at the Lenin Barracks upon his arrival. In his classic account of this experience, Homage to Catalonia, Orwell describes the somewhat comic spectacle of the extremely limited and haphazard training undergone by the new, mostly teenaged militiamen. But there is little comedy in his gritty portrait of the danger, discomfort, cold, and filth of trench warfare that followed.
After three months of such action on the Aragon front at the beginning of 1937, Orwell hoped to join the International Brigade fighting in Madrid which was organized by the Communist International and armed by the Soviet Union. But in May of 1937, the Communist Party staged a propaganda attack on POUM, smearing its leadership and branding its rank and file membership, including Orwell, as Fascists.
The Communist headquarters in Barcelona became a prison for all whom they they deemed Trotskyites, including POUM members. The group's leader, Andreu Nin, was tortured to death by the NKVD. The experience made an indelible impression on Orwell, which he would later draw upon in his evocation of the terror of life in a fictional totalitarian regime.