The price of ingratitude
לא-יבא עמוני ומואבי בקהל יקוק גם דור עשירי לא-יבא להם בקהל יקוק עד-עולם: על-דבר אשר לא-קדמו אתכם בלחם ובמים בדרך בצאתכם ממצרים ואשר שכר עליך את-בלעם וגו’ לקללך
An Ammonite and a Moavite shall not marry into the congregation of Hashem. Even the tenth generation shall not marry into the congregation of Hashem, for all time. Due to the matter that they didn’t present you with bread or water when you were traveling from Egypt, and for having hired Bilaam…to curse you
The Torah informs us that a convert from the nation of Ammon or Moav cannot marry into the Jewish people. The reason is twofold: they didn’t present us with bread or water when we were traveling from Egypt, and because they hired the non-Jewish prophet Bilaam to curse the Jews. If we were to pick the worse of the two crimes, seemingly the second one is more severe. If Bilaam had successfully cursed the Jews, there would be no remnant left. His goal, as well as those who hired him, was to obliterate the Jewish people from the face of the Earth. Shouldn’t that be enough of a reason not to intermarry with them? Why then does the Torah also need to mention the reason that they didn’t offer us bread and water? That was simply a lack of showing honor and respect, or at the very least of generosity. It’s surely not as severe as wanting to annihilate them. Further, why is the sin of not giving bread and water listed first, implying it’s worse than the second one?
Perhaps by mentioning Ammon and Moav’s lack of generosity, the Torah is exacerbating their other sin of hiring Bilaam. Let’s say we were discussing a different nation. If some other nation had hired Bilaam to curse the Jews, it would have been a major crime. However, it wouldn’t have been too great to bear. We know of other nations that have tried to wipe off the Jews from the face of the Earth. Nevertheless, we don’t see any prohibition of marrying their converts. However, it’s different with Ammon and Moav.
Ammon and Moav descend from Lot, Avraham’s nephew. As such, they are our cousins. More than that, Avraham did so much for his nephew Lot. In the civil war between the four and five kings, Avraham risked his life to save Lot, who had been kidnapped. It was due to Avraham that Lot was spared when the city of Sedom was destroyed. He owed Avraham tremendously. As such, his descendants should have been more grateful. They were obligated to pay back the favor, and at the very least provide the Jewish people with the necessary bread and water, as they were weary from their travels.
Not only did Ammon and Moav not act generously with their benefactors, they brazenly paid back the favor by hiring Bilaam to curse them. The intent was to destroy the Jewish people, annihilating them entirely. This extreme level of ingratitude made them not worthy of marrying into the Jewish people. As such, any converts from these nations are forbidden to marry, as well as their descendants, for all time. The Torah doesn’t want us to learn from their horrible character.
Another possible explanation for the two reasons is that the Torah isn’t only explaining why they are forbidden to marry. It’s also explaining why the prohibition lasts for all generations. We see other nations, such as the Egyptians or Edomites, whose converts are also forbidden to marry. However, the third generation onwards are permissible. This is true, despite the horrible acts they’ve done to the Jews. The Egyptians brutally enslaved the Jewish people, torturing them and murdering them. Nevertheless, they benefited the Jewish people. The Egyptians took in Yaakov’s family in their time of need. They allowed them and their descendants to live in their land. As such, we shouldn’t abhor them, and we should allow their third generation to marry into the Jewish people.
If so, if Ammon and Moav had also done something good for the Jews, as in offering them bread and water during their journey from Egypt, the evil they did by hiring Bilaam would have been partially forgiven. It wouldn’t be looked as a totally egregious crime. It could have been that they wouldn’t be forever forbidden from marrying into the Jews. Perhaps, like the Egyptians and Edomites, Ammon and Moav could have married in after a few generations. This makes sense, as Hashem doesn’t withhold reward for any good deed. However, since they didn’t do the Jews any good in all their generations since Lot, and worse than that they hired Bilaam to curse the Jews, they had to be punished. Even their converts are forever forbidden to marry into the Jewish people.
With this, the Torah shows us just how far one small deed can go. No good deed is lost. If they had merely offered the Jews something small like a piece of bread or a cup of water, the ramifications would have been tremendous. Despite their horrific intent and attempt to eradicate the Jewish people, one small deed could have caused it to be forgotten.
 Based on Be’er Yosef to Deuteronomy 23:4,5
 Deuteronomy loc. cit.
 What needs clarification is only Moav hired Bilaam, not Ammon. See note 10
 Parshas Balak, Numbers Chapters 22-24
 See Berachos 7a, based on Micha 6:5
 Genesis 19:30-38
 Ibid 14:14,15
 Ibid 19:29 with Rashi, based on Bereishis Rabbah 51:6
 See Meshech Chochmah to Genesis 15:13 (with Rav Copperman’s notes), who says that non-Jews are prohibited from being ingrates. See also Igros Moshe Yoreh Deah 2:130. This would seem to be because being grateful is a mitzvah sichli, self-evident due to its logic. Some sources which say that non-Jews are obligated in mitzvos sichliyos include Rav Nissim Gaon Introduction to Shas (printed at the beginning of Berachos), Rabbeinu Bachaye to Genesis 18:20, and the Netziv’s approbation to Ahavas Chesed, all brought by Minchas Asher Bereishis § 40. See also Makkos 9b, brought by the Chavos Yair § 166, which shows that mitzvos sichliyos, even though they’re not explicit in the Torah, warrant punishment, even for non-Jews
 The son of the Be’er Yosef cites the Ramban to Deuteronomy 23:5, who seems to give this same explanation, that they should have benefited the Jewish people, and instead tried to curse them. However, the Be’er Yosef takes this a step further, and says that their obligation to repay their debt strengthened the severity of their crime with hiring Bilaam. This is why it states it first. However, a careful read of the Ramban will show that he wasn’t bothered with the Be’er Yosef’s question. The Rambam writes והם גמלו להם רעה, אחד שכר עליו בלעם בן עור והם המואבים, והאחד לא קדם אותו בלחם ובמים. It was Moav who hired Bilaam (as Numbers 22:2-5 says), and it was Ammon who didn’t give food and drink to the Jews. We see then that each one was guilty of a different crime, which is why the verse gives both reasons. This is also the explanation of the Rashba and Ritva to Yevamos 76b s.v. איש דרכו. Worth noting is I saw that Otzaros HaMeforshim (Megillas Rus) Moavi Velo Moavis by Rav Moshe Kravitz writes that a careful reading of the verses will allude to part of this idea. The first reason says אשר לא קדמו, which is in plural. That’s to imply that it’s a crime relevant to both Ammon and Moav (although the Ramban and Rashba say only it was only Ammon; Cf. Akeidas Yitzchak Ki Seitzei § 97 and HaKesav VeHaKabbalah ad. loc. who also make this inference and say it applies to both, and ask this as a question on the Ramban. The latter notes the gemarra itself implies this. See Emes L’Yaakov ad. loc. who tries to answer the Ramban). The second reason says אשר שכר, written in the singular, because it was only Moav that did it
 Deuteronomy 23:8,9
 See Rashi, as well as Rabbeinu Bachaye ad. loc.
 Pesachim 118a