Hi! Here the word "but" means "just", so "but recently" means "just recently/very recently". The sentence is saying that Corsica had been acquired by France very shortly after Napoleon was born and the "very shortly after Napoleon was born" part is shown by using the phrase "but recently".If you remove the phrase "but recently" from the sentence, it will become less emphatic and doesn't mean that conquering Corsica happened shortly after Napolean's birth.