Only after checking the LCL translation and reviewing the grammar of the predicative position did I understand. The translation in English might be something like this:[4] αἰεὶ δὲ ὡς πρὸς εὖ βουλευομένους τοὺς ἐναντίους ἔργῳ παρασκευαζόμεθα. . . .
Translation effort: In actual deeds we prepare, assuming that the adversaries are wise in counsel.
I make this offering hoping that I got it right, and hoping that this short example might help others struggling with the predicative position. Mastronarde IIRC said that the predicative position nearly always implies the verb "to be". I didn't understand his point, but I think I understand it now.