What inspired Orwell to write Animal Farm?

George Orwell was inspired to write this short novel by the Russian Revolution of 1917. Tsar Nicholas II was forced to abdicate his role as Emperor of Russia, and he and his entire family were assassinated a year later by the Bolsheviks. The character of Old Major, the pig who dies early in the story, is inspired by Vladimir Lenin, and Napoleon the pig is Joseph Stalin whereas Snowball the pig is Leon Trotsky.
Nicholas was an incompetent leader who was out of touch with the Russian people and who refused to change with the times, just as Mr. Jones (the farmer who owned Manor Farm) overworked his animals and did not treat them well. The animals rebelled against Jones just as the Russian people, led by Lenin and the Communist Party, rebelled against the tsar. Lenin helped to foment rebellion but then died in early 1924.
Stalin then lead the Communist party in Soviet Russia after Lenin's death until his own death in 1953. Just as Napoleon runs Snowball off the farm, Stalin had Trotsky removed from power in 1928 and eventually banished from the USSR altogether a few months later. Trotsky preferred a democratic socialism that was antithetical to the more dictatorial regime endorsed by Stalin, which is also evident in the ideological conflict between Snowball and Napoleon.