Orwell had been shaped by his experience in the Spanish Civil War and by watching the way the revolution evolved in Russia and then the Soviet Union. He was concerned, as so many in the West were, about the rise of Stalin and what he saw as a "cult of personality" being raised around him. This danger only appeared to increase as Stalin consolidated his power during the second world war.
Orwell himself described Animal Farm as his first effort to use an artistic novel to also try and accomplish a political aim. He was proud of the way he was able to combine the two elements into this very memorable and significant story.
In particular, he felt it was a better representation of Stalin and the Soviet Union than what was generally accepted in Britain at the time he wrote it. He wanted to push back against the very positive image of Stalin held by some leaders and bureaucrats in the government.