Sefer Yona 1:2 Why does Hashem send Yona to tell non-Jews to do teshuva (repentance)? I don't believe this was ever done before or since.
When Nimrod started doing Avoda Zara, the nation of Ashur had no interest in idolatry. The nation of Ashur left their homes and established a new city "Ninve". In that merit Hashem sent Yona to Ninve to tell them to do Teshuva. (Imrei Chein)