ὑπό with the genitive of the personal agent is used (1) when the subject is a person, a city, a country, or is otherwise quasi-personal, (2) when the verb is intransitive even if the subject is a thing, as
τῶν τειχῶν ὑπὸ τῶν βαρβάρων πεπτωκότων
ὡς ἑταίρα_ ἦν . . . ὑπὸ τῶν ἄλλων οἰκείων καὶ ὑπὸ τῶν γειτόνων μεμαρτύρηται
a. νι_κᾶσθαι, ἡττᾶσθαι
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].