The Louisiana Purchase was a scam that went awry. Napoleon "sold it" so he could raise money for his European campaign. The plan was to sell it to the Americans then take it back through force after he successfully conquered Europe.
One of the things I quickly learned after leaving the US is that American historical events are viewed very differently in different sides of the world. Maybe some of the french HNers can give the french perspective.
My understanding is just the interpretation of US history books and one grumpy, old frenchman.