I think the book is perfectly enjoyable as a parable without knowing specifically that it's about Stalin's rise to power.

Frederick is Hitler and his farm is Germany. The animals make an agreement with Frederick, and he reneges on it with fake money - following this he attacks the farm. Same with Hitler, Stalin and the Molotov-Ribbentrop Pact.

The rabbit hole runs deep with the historical parallels in this book.

Characters:

• Boxer is the working class, betrayed by the pigs / Stalin. He does everything he's meant to do loyally but they still turn him into glue

• The four "young pigs" who are killed are likely the top soviets killed during Stalin's Great Purge (Bukharin is the only one I remember)

• Mr Jones is Tsar Nicholas

• Mollie, the horse who leaves because she can no longer get ribbons and sugar cubes, represents the Russian aristocracy who fled after the Revolution

• Moses, with his fairytale talk of "sugarcandy mountain" is the Russian Orthodox Church

• Napoleon's pack of dogs are the KGB

• Old Major is Lenin, whose ideals were corrupted by his successor Napoleon (Stalin). As mentioned in another comment, Snowball is Trotsky - driven out for ideological differences and later turned into an enemy of the state

• The neighbouring farms are the Western powers. Frederick is Hitler - a leader who initially seems aligned with the farm but, it turns out, is betraying it (in real life Operation Barbarossa, in the book the fake money). Pilkington is probably Churchill.

• Mr Whymper is a bit harder, but I think likely to be the kind of useful idiots / tankies that helped spread the soviet agenda in Western countries during the Cold War

Events:

• The hens refusing to give up their eggs is the Ukrainian revolt against collectivism

• The windmill represents Stalin's "five year plan", along with its subsequent failure that leads to famine.

• The Order of the Green Banner is the Order of Lenin

• Frederick / Hitler's attack on the farm is the German invasion of Russia in WW2. It's repulsed, bloodily, just like the USSR threw back the Nazis in places like Stalingrad and Leningrad

The only one I never worked out is the cat. She talks about the revolution supportively but doesn't do any work, and is later found to be playing both sides. I'm not sure if she represents a specific person, or a class.