The famous Taz and the bread of Ammon and Moav[1]
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 וגו’
An Ammonite or a Moabite may not marry into the congregation of Hashem. Even the tenth generation shall not marry into congregation of Hashem, forever. [This is] for the matter in which they didn’t greet you with bread and water as you departed from Egypt…[2]
Although marriage with a non-Jew is forbidden, obviously if someone converted to Judaism they can marry into the fold. However, some nations have restrictions on who or when they can marry. For example, an Egyptian or an Edomite may only marry into the congregation after three generations of being Jewish. In contrast, the Torah forever forbids the nations of Ammon and Moav from marrying into the Jewish people, even if they converted sincerely. Even their descendants are restricted. Why is this? The Torah says it’s because they didn’t greet us with bread and water. The implication is that had they given it to us, we would have been permitted to eat it[3].