Deuteronomy 23:7

Ye shall not seek their peace or, their prosperity all your days, because, if even they become proselytes, they will entertain enmity in their hearts for ever.

THOU SHALT NOT SEEK THEIR PEACE. “Since it is written [with respect to a fugitive slave], He shall dwell with thee, in the midst of thee [where it liketh him best; thou shalt not wrong him], I might think that the rule also applies to this one [an Ammonite or Moabite fugitive slave]. Scripture, therefore, says, Thou shalt not seek their peace.” This is Rashi’s language. Now, this interpretation in the Sifre 251. is deduced from the expression [here] nor their prosperity [Thou shalt not seek their peace ‘nor their prosperity,’ and not as it might appear from the heading in our text of Rashi that it is derived from the expression their peace, for thus is the language of the Sifre]: “Since it is stated there [with respect to the fugitive slave] ‘batov lo’ (where it liketh him best); thou shalt not wrong him — exclude these [the Ammonite and Moabite fugitive slaves] from this tovah (good).” (nor their ‘good’). But the expression Thou shalt not seek their peace is interpreted in the Sifre as follows: “Since it is stated, When thou drawest nigh unto a city to fight against it, then proclaim peace unto it, I might think that the same rule applies to Ammon and Moab. Scripture therefore says, Thou shalt not seek their peace. ” It is also so stated in Tanchuma.. On “Tanchuma” see Vol. IV, p. 158, Note 1.
It appears to me that Scripture has forever forbidden war against Ammon and Moab, and the verse stating, neither contend with them was not only a temporary command, but a negative commandment for the generations. It is for this [reason] that He said, because I have given it unto the children of Lot for a possession., Verse 19. Thus their land belongs to them forever because it is the possession that G-d gave them. If so, the verse here stating, Thou shalt not seek their peace commands that if we wage war against them over a city which they captured from other peoples, that we are not to proclaim peace to them. Similarly, if they invaded our Land we are permitted to pursue them and capture cities from them, to smite the inhabitants of those cities who harmed us, as Jephtah did [of whom it is said], And he smote them from Aroer until thou come to Minnith, even twenty cities. David also did so to all the cities of the children of Ammon because they “breached the fence” first and warred against him. In that event we need not to …