[134] καὶ τῷ μὲν οὐδὲν προὐργιαίτερόν ἐστιν ἢ σκοπεῖν ἐξ ὧν μηδέποτε παυσόμεθα πρὸς ἀλλήλους πολεμοῦντες. . . .
Throughout the speech, but especially in the previous section 133, Isocrates surveys the problem of the Hellenes’ disunity before the power of the Persian king.
Translation: And, in this regard [καὶ τῷ] on the one hand nothing is more important than [for the Persian king] to spy out the causes of which we never stop fighting among ourselves. . . .
I am reading τῷ as having an implied noun denoting the information Isocrates has just given in the preceding section.
I am reading ἐξ ὧν as “the causes of which”. It might be better Englished as “the reasons why”.