Class Division
One of the things that Orwell particularly detests in Britain is the class structure and strictly-adhered to class divisions that make traversing between classes in an upwardly-mobile manner almost impossible. One of the things that draws him to the militia in Spain is the equality that seems to exist across the social spectrum. In Barcelona particularly, when he first arrives, he notes with pleasure that there is no evidence of the previous divisions between classes, and that any references in the language that hint of one class or person being higher than another has been removed - words like "sir" and "madame" are no longer used. He also notes that the working classes are treated the same as the upper classes. This is Orwell's idea of a Utopian society.
However, when he returns to Barcelona just three months later, these changes have been reversed, much to his disappointment. There is once again class division, because it has been impossible to have a successful militia or armed force without it. Without anyone being trained as, treated as or respected as the leader, and therefore superior, there is a directionless rabble that cannot fight. Orwell does not see this and merely bemoans the fact as it is, believing that the ineptitude of the Communist forces has been the cause of this return to the status quo so quickly.
Treachery / Treason
The Communists and POUM had formed an alliance with the purpose of combating Franco and loyalists; however, when Malaga, an important sea port, is taken by the loyalists, who seem to have prior knowledge of the planned attack, it becomes obvious to Orwell and others that the Communists betrayed the left-wing alliance and gave the loyalists warning of what was to come. This kind of treason is thematic in both the book and the war itself. Essentially, it should have been an easily divisible war of two sides, left wing and right wing, but it was not. Each faction had a slightly different objective in their participation in the war, and each realized at different times the obvious conclusion of the war would be a victory to General Franco, thereby feeling it more important politically and strategically to form an alliance of sorts with him, rather than with their idealogical counterparts. Franco would only deal with one of the groups and so betraying their Marxist alliance was the only opportunity the Communists believed they would have to factor in the future of Spain.
Military Ineptiude
The POUM and their alliance are not the best trained force in the world, and certainly not the best armed. Their rifles were practically antiques, over one hundred years old, and so rusted that they would barely fire. Strategic knowledge was minimal. Orwell noted during his first contact with his unit that they were training boys to do the jobs of men, and also that the boys had no understanding of why they were fighting. Lacking in basics in all areas of warfare - strategy, arms and comprehension of the purpose of the exercise - it was almost impossible from the outset for the left-wing alliance to flourish in the war.
Growing Political Cynicism
Over the course of his experiences, Orwell developed a sense of political cynicism that became obvious in his later writings, especially in his novel Animal Farm. He had considered himself to be a Communist but realized that the Communists did not treat the working classes more reasonably than the Fascists or the mainstream parties did, leading him to his "all animals are equal, but some are more equal than others" observation in Animal Farm. He is definitely disillusioned, and this disillusionment is all the more amplified because he had believed so wholeheartedly in the the Soviet machine before the war. This disillusionment turns to political cynicism; the book is scathing about the behavior of the Communists, and the way in which they threw POUM under the bus; it is also savage when it comes to describing the way in which the Fascists treated POUM leadership who were executed without trial. He believes by the end of the book that if the Spanish communists had been better prepared and armed than they might not have been so reliant on Russia, and therefore the outcome of the war might have been different.
