No enemies had ever taken Ankh-Morpock. Well *technically* they had, quite 
often; the city welcomed free-spending barbarian invaders, but somehow the 
puzzled raiders found, after a few days, that they didn't own their horses 
any more, and within a couple of months they were just another minority 
group with its own graffiti and food shops. 
        -- (Terry Pratchett, Eric)