Trouble translating a participle


New Member
I'm reading 46 Stories in Classical Greek by Groton and May and I'm having some trouble on Lesson 24 with the first sentence:

Πολλὰ μὲν οὖν ὑπῆρχε τοῖς ἡμετέροις προγόνοις, ἅτε μιᾶς γνώμης οὖσι, περὶ τοῦ δικαίου μάχεσθαι.

ὑπῆρχε seems to go with μάχεσθαι and since οὖσι is dative plural it must be referring to ἡμετέροις προγόνοις, but the sentence doesn't seem to make sense regardless. Am I missing something?

EDIT: Completely missed that μιᾶς γνώμης is genitive, making it mean "It was very natural to our ancestors, being of one will, to fight concerning justice." This thread can be deleted.