Symbolism, Imagery, Allegory
Napoleon is obviously a real guy, but he's important to the novel because he hugely influences the plot without ever showing up in the world of Becky, Amelia, Rawdon, and George. We'd like to suggest that he's a kind of double symbol.
On the one hand, we have Becky-as-Napoleon. Napoleon works as a kind of emblem of limitless, relentless, inevitable conquest. Becky is often compared to "the Corsican upstart" (Napoleon) and Thackeray frequently describes her actions in military terms. She is always invading or attacking or routing the person she's dealing with. What do we make of this comparison? Is Becky really the Napoleon of her social world?
On the other hand, we've got Napoleon-as-fate. Think about how wide-reaching the effects of his escape from Elba and return to the battlefield are on the people in the novel. Of course his return summons George and Rawdon to fight in Brussels, but even before that it upsets the financial markets Mr. Sedley depends on, thus wrecking his relationship with Mr. Osborne and affecting Amelia's marriage.
Thackeray spends a paragraph teasing out this chaos theory idea, the notion that a butterfly flapping its wings could cause a tsunami a thousand miles away. In this case, it's the reverse:
[…] is it not hard that the fateful rush of the great Imperial struggle can't take place without affecting a poor little harmless girl of eighteen [...] Yes; Napoleon is flinging his last stake, and poor little Emmy Sedley's happiness forms, somehow, part of it. (18.2)