Why does Napoleon try to rewrite history in order to further his own ends as well as banish Snowball with no justification in Animal Farm?

This novel presents us with history as a flexible entity,
that is open to manipulation and change to suit the ends of Napoleon and his cronies,
the other pigs. Especially after the expulsion of Snowball from the farm, we see that
history is re-written again and again to maintain control and allow Napoleon to adjust
his rule to various and changing circumstances. Note the way in which, almost straight
after Snowball leaves, Napoleon suddenly changes his mind concerning the Windmill, but
of course, this is not presented this way:

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That evening Squealer explained privately to the
other animals that Napoleon had never in reality been opposed to the windmill. ON teh
contrary, it was he who had advocated it in the beginning, and the plan which snowball
had drawn on the floor of the incubator shed had actually been stolen from among
Napoleon's papers. The windmill was, in fact, Napoleon's own
creation.

The "truth" is
constantly re-written throughout the novel, and, as your question says, this is to
ensure that Napoleon maintains his own power and does not present himself as weak,
vacillating leader that changes his plans and strategy.