Rashi on Numbers 21:26/רש"י על במדבר כ״א:כ״ו

1והוא נלחם AND HE (SIHON) HAD FOUGHT [AGAINST THE FORMER KING OF MOAB] — Why is it necessary that this should be written? Because it is said (Deuteronomy 2:9), “Harass not Moab”, and Heshbon belonged to Moab (and therefore should not have been attacked), it (Scripture) tells us that Sihon had taken it from them, and through him it (Heshbon) became permissible to Israel as an object of attack (Chullin 60b; Gittin 38a).

2מידו means FROM HIS POSSESSION (Bava Metzia 56b).