Much like Stalin, Napoleon erred in making the decision to do business with Mr. Frederick. The stack of five-pound notes Napoleon received turned out to be counterfeits, even though the pigs laud Napoleon's genius in brokering the deal. And, like Hitler, Frederick and a group of men attacked the farm shortly after the agreement, dynamiting the windmill before being driven off by the enraged animals.
This represented Stalin's choice to go with Hitler in the years before World War II. Stalin, of course, is represented by Napoleon. In real life, Stalin eventually made a pact with Hitler in which he and Hitler agreed to refrain from attacking one another. This was, eventually, broken by Hitler when he invaded the Soviet Union.