Ammon as opposed to Moav and Midyan

Shalom and thank you for your question! It is an excellent one. In what way did Ammon cause the Israelites to sin…

On the face of it the Torah text is indeed unclear as to what in the differentiation between Ammon and Moav, and why Ammon is quoted by Rashi as having caused the Israelites to sin.

However if we look a little further we shall see.

Ammon and Moav as you may recall came from Lot. They were born because their mothers,  Lots daughters,  thought that all civilization except for they and their father had been destroyed. Therefore they reasoned that they should get pregnant from their father and populate the world. So Ammon and Moav were brothers, and continued a close relationship as they grew into tribes. They lived in close geographic proximity. The Torah tells us , as you point out,  that they did not come out towards him with bread and water, and it says “on the matter that they did not come forward to you with bread and water.”

Rashi explains that this word ‘על דבר’ in Hebrew ‘on the matter ‘, (or the word,) indicates that the Torah is referring to the IDEA they had to make us sin. And he continues to explain, this was when we were traveling and thus in a non-settled and vulnerable state. The commentary of the Sfas Emes concurrs with this approach and takes it further. He maintains that this matter, or word, or idea, was not apparent on the surface, but Ammon and Moav actually took counsel together to cause the Israelites to sin. So Ammon was a silent partner if you like. Their aim was definitely to weaken the Jewish people spiritually so that they would fall prey to sin and thus not merit Divine protection. They intended to offer them non-kosher food as well as draw them to idol worship through promiscuity with their daughters.

We hope this answers your question!

May you be inscribed for a good and sweet year!