John Gill Commentary Psalms 136:17

"To him that smote great kings; For his lovingkindness [endureth] for ever;" — Psalms 136:17 (ASV)

To him which smote great kings
Either the thirty one kings in the land of Canaan; or those who are after particularly mentioned; which seems best, since the kings of Canaan were petty kings, when these were greater ones;

for his mercy [endureth] for ever ;
the smiting of those kings were in mercy to Israel; both to preserve them from falling into their hands, and that they might possess their countries, as is later declared.