He gained leadership Dictatorship over all the animals on Animal Farm. Not to mention that Squealer warped the memories of the other animals by tricking them into believing the lies he spun. The dogs enforced all the ridicules "laws" made by Napoleon.
Napoleon banished snowball because he was trying to tell the animals what was really going on and napoleon wanted to be a tyrant while snowball was trying to keep that from happening
So when the events on Animal Farm deteriorated, Napolean and his veegs would have an easy soft target to deflect the blame from their own ineptness and poor planning.
Napoleon orders death for anyone who gives the hens food. They are rebelling because they don't want Napoleon to sell their eggs.
Napoleon convinces the the animals that all the unfortunate instances on Animal Farm were because of Snowball. He propagandized Snowball to be the bad guy and used him as a scapegoat in order to gain control. Napoleon also used the dangerous dogs to intimidate the other farm animals into not questioning him. All in all, Napoleon used power and fear the oust Snowball.
Snowball wanted to spread Animalism instead, arguing that defence was not needed if Animalism was widespread. However, Napoleon believed that Animal Farm's defence was more important that the spreading of Animalism, at that point of time, because the humans may attack them.
He invaded it with his armies.
Napoleon privately rears some puppies that he took away from their mothers when they were young. He trained them to be vicious killing machines - and in chapter 5, he uses them to chase off snowball and to intimidate the other animals in order to gain power.
Plebiscite vote
Napoleon uses many tools of propaganda to gain power. His main tool perhaps were the sheep. Whevever any of the other animals would want to speech up the sheep broke out in to "four legs good, two legs bad". However, Napolean only ever gained complete power from exiling snowball. He knew from the windmill vote that snowball was a threat, and found it very easy to get the animals to side with him. So using teh dogs, which he trained in secret, he got rid of this danger.
Napoleon is not the protagonist, but is a main character who represents corruption. Napoleon twists the rules and morals of his cause for his own gain, and is shown as an antagonist and foil to Snowball.
He uses his 9 dogs
He was well paid for his service to France.
Aided industry trough tariffs and loans