What made the building of the windmill controversial would, according to Snowball, be Napoleon's rejection of the whole concept. The whole idea of building the windmill would be to benefit ALL the animals on the farm. Having it would mean they could generate electricity and make work easier. They would be able to use electrical machinery and, therefore, reduce the workload for everyone, as Snowball explained in chapter 5:
Snowball declared that this was just the place for a windmill, which could be made to operate a dynamo and supply the farm with electrical power. This would light the stalls and warm them in winter, and would also run a circular saw, a chaff-cutter, a mangel-slicer, and an electric milking machine. The animals had never heard of anything of this kind before (for the farm was an old-fashioned one and had only the most primitive machinery), and they listened in astonishment while Snowball conjured up pictures of fantastic machines which would do their work for them while they grazed at their ease in the fields or improved their minds with reading and conversation.
Snowball's plan would be in line with the ideas contained in Animalism, which was created to make the lives of all animals easier. The purpose of the Rebellion was, after all, to improve the lot of the animals, and this was exactly what Snowball set out to do. It is for this reason that he would obviously not understand why Napoleon opposed his grand idea, and this is what created the controversy.
The whole issue would have been better understood if Napoleon had come up with an opposing idea, but he never produced any schemes of his own. This must have been very confusing for Snowball, since he had everyone's best interests at heart. Be that as it may, Napoleon declared himself against the windmill and showed his disgust for Snowball's hard work by urinating all over his plans.
Because of Napoleon's opposition, the entire farm was deeply divided over the issue, for both he and Snowball had their own followers and two factions had formed, each carrying a slogan favoring either Snowball or Napoleon. The matter had to be resolved and would be put to a vote at the next meeting. Snowball spoke eloquently, while Napoleon only mentioned that he hated the idea and did not want anyone to vote for it. It became clear after another emphatic speech by Snowball that he would easily win the vote.
It was at this point that Napoleon played his trump card. He called on the dogs, which he had secretly raised since they had been puppies, to attack Snowball, and they chased him off the farm.
This incident made it clear what Napoleon had slyly been plotting all along. He had always resented Snowball's intelligence and eloquence and saw him as competition for the ultimate leadership of the farm. The vote gave him an opportunity to exercise his brutal lust for power. Getting rid of Snowball meant that he would, in future, have no opposition.
Snowball, on the other hand, was too naive and good-natured to figure out what Napoleon ultimately had in mind. He probably believed that since they were all in it together, they would be of one mind about how to improve the animals' fates. It is here that he was wrong, for Napoleon had been thinking only about himself and getting more power.