The main point of Animal Farm is about the nature of power. At first, the revolution had noble intentions. The communist ideals, seen in the seven commandments, sound great in theory. However, in time the leaders on the farm are not able to follow those ideals with any consistency. As the story progresses, they follow the ideals less and less. Towards the end, they completely betray any ideals. This is precisely why at the end of the book, Orwell has the pigs turn into men.
Clover cannot see who is man or pig, or pig and man. Here are the words of Orwell:
No question, now, what had happened to the faces of the pigs. The creatures outside looked from pig to man, and from man to pig, and from pig to man again; but already it was impossible to say which was which.
From this quote it seems clear that the main theme is about how power corrupts. To ground Animal Farm in history, we can say that even though Orwell believed in communist ideals, he believes that the Soviet Union, under Stalin, implemented these ideals in a perverse and wicked form.