Deuteronomy 23:4 "An Ammonite or a Moabite shall not enter the congregation of G‑d"
We are not allowed to marry a man from the nations of Ammon or Moab even if he has converted to Judaism.
However, it is permissible to marry a woman from Ammon or Moab after she has converted to Judaism.
These nations refused to offer bread and water to the Jewish people when they passed by their borders and because of this act of cruelty, they were excluded from ever being able to join the Jewish people in marriage.
To Jose: Even today, if a nice Jewish girl would meet a guy who would trace his roots back to Amon or Moab, she would not be allowed to marry him—even if he converted to Judaism. The fact is, however, that there are no such people.
So how can we apply this to our daily lives? Let us look at a similar mitzvah: to destroy the seed of Amalek. While this mitzvah has ceased to have much practical implication (for the time being), it surely does have a very important spiritual legacy, and that is to eradicate the Amalek within each of us, the cynical voice which tells us to not to be such a faithful follower of the Torah.
In light of the above, do you think that there can be a similar application for staying away from Amon and Moab?