Albert Barnes Commentary Zechariah 6:7

"And the strong went forth, and sought to go that they might walk to and fro through the earth: and he said, Get you hence, walk to and fro through the earth. So they walked to and fro through the earth." — Zechariah 6:7 (ASV)

And the strong went forth and sought to go, that they might walk to and fro through the earth - The mention of their strength corresponds to the extent of the power and commission for which they asked: to go to and fro, up and down, at their will, unhindered, through the whole earth. The Babylonian empire held Egypt as its only territory outside of Asia; the Persian empire was conquered in its efforts against Europe, in Greece; Alexander's empire was like a meteor, gleaming but breaking into four parts; the Roman empire combined East and West and, within its large limits, ruled tranquilly.

And he said, Go, walk to and fro in the earth - He commanded, and they, who were previously withheld, went, and they walked to and fro on the earth, ordering all things at their will, under the Providence of God, by which He gave free access to the Gospel throughout their wide empire. After the Greek empire was extinguished, the Romans no longer merely went into any particular country, but superintended and governed all human affairs in (it is the language of the New Testament) all the world. Cyril: “These same, the dappled and ashen-grey horses were commanded to traverse the earth, and they did traverse it; for they mastered all under heaven and ruled the whole earth, God consenting and arraying those who wielded the Roman might with this brilliant glory. For, as God, He knew beforehand the greatness of their future piety.”