Arise, O Lord (comp. Numbers 10:35; Psalms 7:6; Psalms 9:19; Psalms 10:12; Psalms 17:13; Psalms 68:1). This call is generally made when God's forbearance towards his enemies is thought to have been excessive, and his tolerance of sinners too great.
Save me, O my God. David was in imminent danger. "All Israel" had come against him (2 Samuel 16:15). He was short of supplies (2 Samuel 17:29). He was doubtful how God was disposed towards him (2 Samuel 15:25, 2 Samuel 15:26).
It was a time when, unless God would save, there could be no hope. Hence the intense earnestness of his prayer. For thou hast smitten all mine enemies upon the cheek-bone. Heretofore, i.e; thou hast always taken my part—thou hast smitten mine enemies, and given me victory over them, and by breaking their jaw-bones thou hast taken away from them all power to hurt (see Psalms 58:6).
The reference is, of course, to David's long series of victories, as those over the Philistines (2 Samuel 5:17-25; 2 Samuel 8:1), over Moab (2 Samuel 8:2), over Hadadezer, King of Zobah (2 Samuel 8:3, 2 Samuel 8:4), over the Syrians of Damascus (2 Samuel 8:6), over the Edomites (2 Samuel 8:13, 2 Samuel 8:14), over the Ammonites (2 Samuel 10:7-14), and over the "Syrians beyond the river" (2 Samuel 10:16-19).
Thou hast broken the teeth of the ungodly (comp. Job 4:10; Psalms 58:6). The ungodly, enemies alike of David and of God, are represented as wild beasts whose weapons are their jaws and teeth. Let God break these, and they are harmless.