Charles H. Spurgeon on Psalms 136:17

To him which smote great kings. Within sight of their inheritance Israel had to face powerful enemies. Kings judged to be great because of the armies at their back blocked up their road. This difficulty soon disappeared, for the Lord smote their adversaries, and a single stroke sufficed for their destruction. He who had subdued the really mighty ruler of Egypt made short work of these petty sovereigns, great though they were in the esteem of neighbouring princes. For his mercy endureth for ever. Mercy, which had brought the chosen tribes so far, would not be balked by the opposition of boastful foes. The Lord who smote Pharaoh at the beginning of the wilderness march, smote Sihon and Og at the close of it. How could these kings hope to succeed when even mercy itself was in arms against them.

Evermore his mercy stands

Saving from the foe man's hands.