The next time you reach into your pantry for a bottle of Italian extra virgin olive oil, take a look at the ingredients on the back label. Chances are it contains a blend of oils of Mediterranean origin, with a heavy emphasis on Spain. It’s no wonder, given that Europe’s fourth-largest country produces 45% of the world’s “liquid gold,” as Homer memorably dubbed olive oil in his epic the Iliad. Why that Spanish oil got relabeled as Italian is a story that spans centuries and many thousands of...