My French course (Rosetta Stone) has the following examples:
- Cet homme est d’Australie
- Cet homme est de France
- Cet homme est de Russie
- Cet homme est du Japon
Why the other three just use “de” but Japon has to use “du”?
Masculine vs feminine. "de" + feminine country name vs "du" [de+le] + masculine country name.
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.