Napoleon convinces the other animals that Snowball is the cause of all their troubles. He makes sure that the other animals believe that Snowball was a traitor and he was going to help the farmer get the farm back.
“Our Leader, Comrade Napoleon…has stated categorically…that Snowball was Jones’s agent from the very beginning…and from long before the Rebellion was ever thought of.”
Napoleon makes it seem to the other animals that Snowball was against them and blames all their hardships on his traitorous behavior.
Napolean uses the fear factor with the other animals on the farm when he implicates Snowball. This is very much like what Hitler did with the Jews in WWII. Hitler was able to convince Germany that the Jews were a threat. By creating this sense of fear on the farm Napolean keeps the animals on his side. I am sure you have been made aware that Animal Farm is an allegory for the Russian Revolution. Snowball is Leon Trotsky. Do some research on what happened to Trotsky, this will answer your question even further.