This psalm likewise purports to be "A Psalm of David," and there is no cause to doubt the correctness of the superscription which ascribes it to him. There is, however, no certain intimation at what time of his life, or on what occasion, it was composed, and it is impossible to determine these points.
The most probable supposition in regard to its composition seems to me to be, that it is a song of thanksgiving for the victory secured in answer to the prayer of himself and the people in the previous psalm. Nothing can be argued, indeed, on this point, from the mere fact that it stands in close connection with the previous psalm; but there are, it seems to me, internal marks that this was its design, and that it is the expression of a heart overflowing with gratitude, and, therefore, recalling not merely the immediate blessings of a recent victory, but also the other blessings with which God had crowned his life, Psalm 21:3-4.
Thus understood in regard to its origin, the psalm may be regarded as divided into the following parts:
I. Thanksgiving for success, or for granting the object which had been so earnestly sought, Psalm 21:1-7. In this thanksgiving the psalmist says that God had not only granted what had been asked Psalm 21:1-3, but that he had greatly "exceeded" this: he had granted far more than had been the literal request. He had added blessings which had not been specifically sought; he had made those blessings permanent and eternal, Psalm 21:4-7.
II. The general truth that "all" the foes of God would thus be overcome, and that the cause of truth would be finally triumphant, Psalm 21:8-12. This was "suggested" by the victory which had been achieved. As God had granted that victory, as he had so easily subdued the enemies of himself and of his people - as he had gone so far beyond the expectations and the hopes of those who had gone forth to the conflict, the idea is naturally suggested that it would be thus with all his foes, and that there would be ultimately a complete victory over them.
III. The expression of an earnest "desire" that God might be thus exalted, and might thus achieve a complete and final victory, Psalm 21:13,
For the meaning of the phrase, "To the chief Musician," in the title to the psalm, see the notes at Psalm 4:1-8.

The psalmist returns thanks to God for giving him the victory over his enemies; which victory he had earnestly requested, Psalm 21:1, Psalm 21:2. He enters into a detail of the blessings that in consequent of the victory he had obtained, Psalm 21:3-7. He predicts the destruction of all those who may hereafter rise up against him, Psalm 21:8-12; and concludes with praising the power of Jehovah, Psalm 21:13.
In the title of this Psalm there is nothing particularly worthy of remark. The occasion of it is variously understood. Some think it was composed to celebrate the victory obtained over Sennacherib; others, that it was made on the recovery of Hezekiah, and the grant of fifteen years of longer life; see Psalm 21:4. Others and they with most appearance of propriety consider it a song of rejoicing composed by David for his victory over the Ammonites which ended in the capture of the royal city of Rabbah, the crown of whose king David put on his own head, see Psalm 21:3, and to procure which victory David offered the prayers and sacrifices mentioned in the preceding Psalm. Lastly, many think that it is to be wholly referred to the victories of the Messiah; and it must be owned that there are several expressions in it which apply better to our Lord than to David, or to any other person; and to him the Targum applies it, as does likewise my old Anglo-Scottish Psalter in paraphrasing the text.

INTRODUCTION TO PSALM 21
To the chief Musician, a Psalm of David. This psalm was either written by David; and therefore called a "psalm of David"; or it was written, as it may be rendered, "for David", by some other person, on account of his victories and triumphs; or rather "concerning David" (s); that is, concerning the Messiah, the son of David, as Kimchi says some expound it; or concerning the Messiah, who is called David, Ezekiel 37:24; and Jarchi observes, that their Rabbins interpret it of the Messiah; but, says he, it is right to explain it, moreover, of David himself, for an answer to the heretics (Christians) who err in it; and various passages in this psalm are by the Jewish writers understood of the Messiah; as "the King", in Psalm 20:1 is in the Targum called the King Messiah; Psalm 21:4 is in the Talmud applied (t) to him; Psalm 21:3 are in Zohar (u), and in the Midrashes (w), interpreted of him; and many Christian writers understand the whole of him; which is right: though Theodoret thinks it was penned on the account of the health of King Hezekiah, and his restoration from his disease; which is not likely.
(s) "pro Davide, vel de Davide", Vatablus. (t) T. Bab. Succah, fol. 52. 1. Vid. Nachman. disput. "cum fratre Paulo", p. 36. Ed. Wagenseil. (u) In Numb. fol. 68. 3. 4. (w) Midrash Tillim apud Viccars. in loc. & in Galatin. l. 3. c. 9. Bemidbar Rabba, fol. 212. 4. & 218. 1.

(Psalm 21:1-6) Thanksgiving for victory.
(Psalm 21:7-13) Confidence of further success.

Thanksgiving for the King in Time of War
"Jahve fulfil all thy desires" cried the people in the preceding Psalm, as they interceded on behalf of their king; and in this Psalm they are able thankfully to say to God "the desire of his heart hast Thou granted." In both Psalm the people come before God with matters that concern the welfare of their king; in the former, with their wishes and prayers, in the latter, their thanksgivings and hopes in the latter as in the former when in the midst of war, but in the latter after the recovery of the king, in the certainty of a victorious termination of the war.
The Targum and the Talmud, B. Succa 52a, understand this Psalm 21:1 of the king Messiah. Rashi remarks that this Messianic interpretation ought rather to be given up for the sake of the Christians. But even the Christian exposition cannot surely mean to hold fast this interpretation so directly and rigidly as formerly. This pair of Psalm treats of David; David's cause, however, in its course towards a triumphant issue - a course leading through suffering - is certainly figuratively the cause of Christ.