Masculine vs feminine. “de” + feminine country name vs “du” [de+le] + masculine country name.

• du Japon, du Gabon, du Pérou

NB some countries are not used with an article (so technically they have no gender), like Cuba or Puerto Rico. In which case, for “neutral” country names, it is “de + name”. De Cuba, de Guernesey.

Finally, for country names that are used with the plural form, like Etats Unis, or Caraïbes, it is [de + les] = des. Des Etats Unis, des Caraïbes.