Rashi on Deuteronomy 2:9/רש"י על דברים ב׳:ט׳

1ואל תתגר בם וגו׳ AND DO NOT CONTEND WITH THEM [IN WAR] — As regards Moab He forbade them (the Israelites) only war against them, but they may frighten them, appearing before them when equipped for war; therefore it is written, (Numbers 22:3) "And Moab was afraid because of the people", because they took plunder and loot from them. But about the children of Ammon it is said, (v. 19) “Do not contend with them" — no provocation of any kind, — as a reward for the modesty shown by their ancestress, Lot's younger daughter, who did not publicly divulge regarding her father's conduct, as his elder daughter did, who called her son's name “Moab" (i.e. born of the father) (Bava Kamma 38b; see Rashi on Genesis 19:37).

2ער is the name of the district.