In my book, Napoleon (whose surname is Bonaparte) is referred to as Buonaparte. There is a scene in the book where a diplomat says they should remove the u and call it Bonaparte, but that joke couldn't have gone so far that Wikipedia refers to him as Bonaparte. So, here are my two questions, how did the name Bonaparte be used, and why was he referred to as Buonaparte?
Napoleon Bonaparte was born as Napoleone Buonaparte into a corsican family with ligurian, i.e. Italian, roots. The extra 'e' and 'u' were just due to the way his name was written in Italian, in corsican his name was even Nabulione.
While the B(u)onapartes had actually taken part in a partisan war against the French takeover of Corsica after it had been sold by Genua, Napoleon's father Carlo Buonaparte ultimately gave in to the French power and accepted the French rule, becoming a French (lower) nobleman.
And despite the fact of his nobility as well as the fact the he was priviliged and had benefitted from his social status Napoleon welcomed the French Revolution and pledged allegiance to the new system, hence transferring from the royal to the revolutionary army.
In march 1796 he married Josephine de Beauharnais. When he was on the way to the war in Italy shortly after their wedding he wrote her a letter which is the first known instance where he signed with Napoleon Bonaparte, which is usually seen as the point where he had definitively decided that he was actually French.
That the European Nobility still used the Italian version of his name long after he had changed it is of course to deminish him in their minds. He wasn't of real nobility, he was just a squire, and at that he wasn't even French but some Italian chawbacon from insignificant backwood Corsica. We see the same principle at work when Hitler is referred to as "Schicklgruber" or "the bohemian private".