Napoleon's decision to accept the windmill project farm is a tactic or a method that he uses to control the farm and the animals. When Napoleon decided that Snowball was getting too popular and perceived him as a threat to his power, he decided to get rid of him. This shocked the animals at first, who did not understand why Snowball was chased off the farm by Napoleon's dogs.
However, then Squealer begins to persuade the animals that Snowball was really a traitor and not part of the rebellion. He says that the idea for the windmill was actually Napoleon's and that Snowball actually stole the idea. Squealer also tried to convince the animals that Napoleon was all in favor of the windmill so as to keep them calm and reassured, believing that they were all working for the betterment of the farm.
It was a tactic, or management of a situation method that he employs to make the animals think that Napoleon has their best interests at heart. Napoleon does not care about the windmill, he cares about keeping the animals so busy that they don't have time to talk to each other and question what is going on the farm.