A few factual corrections to dogwalker's otherwise excellent write-up:
The POUM were not "a group of rebels fighting against the Stalinist Spanish government." The Spanish Civil War started as a right-wing military revolt against the Popular Front government: the POUM and the Stalinist Communists were both fighting to defend the government, at least at first.
Orwell left the BBC in 1943. not 1945. He was Literary Editor of Tribune, a (then) socialist political magazine, between 1943 and 1945.