Albert Barnes Commentary Psalms 68:28

"Thy God hath commanded thy strength: Strengthen, O God, that which thou hast wrought for us." — Psalms 68:28 (ASV)

Thy God hath commanded thy strength - Has ordered your strength to appear, or to be manifested. This is addressed, evidently, to the people of the land; and the idea is that, on this occasion, God had called forth a full representation of the strength of the nation; or, as we would say, there had been a full “turn out.” It was an impressive sight, showing the real strength of the people.

Strengthen, O God, that which thou hast wrought for us - Increase the strength thus manifested. Let it be still greater. The scene is now impressive and grand; make it still more so, by adding to the number and the prosperity of your people. This is an illustration of the desire in the heart of every pious person that, whatever prosperity God may have given to his people, he would give a still larger measure—that however greatly he may have increased their numbers, he would add to them many more. This desire of the heart of piety will not be satisfied until the whole world is converted to God.