What events lead to Napoleon's seizing complete control of the farm, and which different tactics does he use to succeed in Chapter V?
From the beginning of Chapter II, Napoleon begins to assume control of the farm, as he and the other pigs devise and impose a series of seven commandments which dictate the behavior of the other animals. It is not until Chapter V, however, that Napoleon makes his move to assert complete control over the farm. He accomplishes this through a series of "diplomatic" moves.
Snowball, Napoleon's only real political adversary on the farm, approaches him with a design for a windmill he would like to see on the farm. Napoleon believes the construction of the windmill would divert the animals' attention from what is truly important - the production of food. In the political battle that ensues, Napoleon makes a power play by bringing in nine dogs who are loyal to him and having them remove Snowball from the farm. The tactic Napoleon uses in this situation is quite straightforward; however, what he does to "smooth things over" with the other animals says much more about his political methods and goes further to show his political savvy.
To justify his removal of Snowball from the farm, Napoleon has Squealer, his "information officer," begin the rumor that Snowball is a criminal, giving the impression that Napoleon's actions were in the service of the "state." To further this idea, Napoleon agrees to build the windmill on the farm, all the time asserting it was his idea from the beginning. While these "moves" are intended to demonstrate Napoleon's dedication to the state, they also serve to ensure greater loyalty from the other animals. Any criticism against Napoleon would, by extension, be a criticism against the state.