(Devarim 23:7) "Do not seek their peace and their good all of your days, forever": Because it is written (Ibid. 20:10) "and you shall call out to it (a besieged city) for peace," I might think that the same should be done with them (Amon and Moav); it is, therefore, written "Do not seek their peace."
And because it is written (Ibid. 23:17) "… where it is good for him (an escaped servant); you shall not oppress him," I might think that the same holds true with them; it is, therefore, written "and (do not seek) their good."
"all of your days, forever": for ever and ever.