This psalm purports to be "A Psalm of David," nor is there any reason to doubt that he wrote it. Of the precise occasion on which it was composed nothing can be known with certainty, for there is no historical statement on the point, and there is nothing in the psalm to indicate it. It would seem, however, from the psalm, that it was composed on some occasion when the king was about going to war, and that it was designed to be used by the people of the nation, and by the king and his hosts mustered for war, as expressing mutually their wishes in regard to the result, and their confidence in each other and in God. Or if it was not designed to be used by the people actually, it was intended to be a poetic expression of the real feelings of the king and the people in regard to the enterprise in which he was embarked.
According to this idea, and as seems to me to be manifest on the face of the psalm, it is composed of alternate parts as if to be used by the people, and by the king and his followers, in alternate responses, closing with a chorus to be used by all. If it was intended to be employed in public service, it was doubtless to be sung by alternate choirs, representing the people and the king.
The whole may be divided into three strophes or parts:
I. The first strophe, Psalm 20:1-5.
(a) the people, Psalm 20:1-5. They pray that the Lord would defend the king in the day of trouble; that the name of the God of Jacob would defend him; that he would send him help from the sanctuary, and strengthen him out of Zion; that he would remember his offerings and accept his burnt sacrifice; that he would grant him according to his own heart, and fulfill all his counsel.
(b) the king, Psalm 20:5, first part. He says, as expressive of the feeling with which the expedition was undertaken, "We will rejoice in thy salvation, and in the name of our God we will set up our banners."
II. The second strophe, Psalm 20:5 (latter part), and Psalm 20:6.
(a) the people, Psalm 20:5, latter clause; expressing a desire for his success and triumph, "The Lord fulfil all thy petitions."
(b) the king, Psalm 20:6; expressing confidence of success from the observed zeal and cooperation of the people: "Now know I that the Lord sayeth his anointed; he will hear him from his holy heaven with the saving strength of his right hand."
III. general chorus of all, Psalm 20:7-9. This is the language of exultation and triumph in God; of joyful trust in him. "Some," is the language of this chorus, "trust in chariots and some in horses, but we will remember the name of the Lord our God," Psalm 20:7. Then they see their enemies fallen and subdued, while their armies stand upright and firm, Psalm 20:8. Then they call, in joyful exultation and triumph, on God as the great King over all, and supplicate his mercy and favor, Psalm 20:9.
This is, therefore, a patriotic and loyal psalm, full of confidence in the king as he starts on his expedition, full of desire for his success, and full of confidence in God; expressing union of heart between the sovereign and the people, and the union of all their hearts in the great God.
On the meaning of the phrase in the title, "To the chief Musician," see the note at the title to Psalm 4:1-8.

A prayer for the king in his enterprises, that his prayers may be heard, his offerings accepted, and his wishes fulfilled, Psalm 20:1-4. Confidence of victory expressed, Psalm 20:5, Psalm 20:6. Vain hopes exposed; and supplication made for the king, Psalm 20:7-9.
It is most likely that this Psalm was penned on the occasion of David's going to war, and most probably with the Ammonites and Syrians, who came with great numbers of horses and chariots to fight with him. See 2-Samuel 10:6-8; 1-Chronicles 19:7. It is one of the Dialogue Psalm, and appears to be thus divided: Previously to his undertaking the war, David comes to the tabernacle to offer sacrifice. This being done, the people, in the king's behalf, offer up their prayers; these are included in the three first verses: the fourth was probably spoken by the high priest; the fifth, by David and his attendants; the last clause, by the high priest; the sixth, by the high priest, after the victim was consumed; the seventh and eighth, by David and his men; and the ninth, as a chorus by all the congregation.

INTRODUCTION TO PSALM 20
To the chief Musician, a Psalm of David. This psalm is thought, by some, to be written by David, on account of himself, and as a form to be used by the people for him, when he was about to go to war; particularly with the Ammonites and Syrians, 2-Samuel 10:6; mention being made of chariots in it, Psalm 20:7; of which there was a great number in that war: Arama thinks it was made by him when he got the victory over the Philistines; others think it was written by one of the singers on David's account, and should be rendered, "a psalm, for David", as Psalm 72:1, but rather it is a psalm concerning David; concerning the Messiah, whose name is David; or a psalm of David concerning the Messiah, since he is expressly mentioned, Psalm 20:6; and Aben Ezra says, there are some that interpret it of the Messiah; and some passages in it are, by Jewish writers (m), applied unto him, as Psalm 20:6; and our countryman, Mr. Ainsworth, says, the whole psalm is a prophecy of Christ's sufferings, and his deliverance out of them, for which the church with him triumphs. Theodoret takes it to be a prophecy of Sennacherib's invasion of Judea, and of Rabshakeh's blasphemy, and of Hezekiah's distress and prayer on that account.
(m) Shirhashirim Rabba, fol. 18. 1. Tzeror Hammor, fol. 44. 2.

This psalm is a prayer for the kings of Israel, but with relation to Christ.

Prayer for the King in Time of War
To Psalm 19:1-14 is closely attached Psalm 20:1-9, because its commencement is as it were the echo of the prayer with which the former closes; and to Psalm 20:1-9 is closely attached Psalm 21:1-13, because both Psalm refer to the same event relatively, as prayer and thanksgiving. Psalm 20:1-9 is an intercessory psalm of the nation, and Psalm 21:1-13 a thanksgiving psalm of the nation, on behalf of its king. It is clearly manifest that the two Psalm form a pair, being connected by unity of author and subject. They both open somewhat uniformly with a synonymous parallelism of the members, Psalm 20:2-6; Psalm 21:2-8; they then increase in fervour and assume a more vivid colouring as they come to speak of the foes of the king and the empire, Psalm 20:7-9; Psalm 21:9-13; and they both close with an ejaculatory cry to Jahve, Ps 20:10; 21:14. In both, the king is apostrophised through the course of the several verses, Psalm 20:2-6; Psalm 21:9-13; and here and there this is done in a way that provokes the question whether the words are not rather addressed to Jahve, Psalm 20:6; Psalm 21:10. In both Psalm the king is referred to by המּלך, Ps 20:10; Psalm 21:8; both comprehend the goal of the desires in the word ישׁוּעה, Psalm 20:6, cf. Psalm 20:7, Psalm 21:2, Psalm 21:6; both delight in rare forms of expression, which are found only in these instances in the whole range of Old Testament literature, viz., נדגל Psalm 20:6, נתעדד Psalm 20:9, ארשׁת Psalm 21:3, תחדהו, Psalm 21:7.
If, as the לדוד indicates, they formed part of the oldest Davidic Psalter, then it is notwithstanding more probable that their author is a contemporary poet, than that it is David himself. For, although both as to form of expression (cf. Psalm 21:12 with Psalm 10:2) and as to thoughts (cf. Psalm 21:7 with Psalm 16:11), they exhibit some points of contact with Davidic Psalm, they still stand isolated by their peculiar character. But that David is their subject, as the inscription לדוד, and their position in the midst of the Davidic Psalm, lead one to expect, is capable of confirmation. During the time of the Syro-Ammonitish war comes David's deep fall, which in itself and in its consequences made him sick both in soul and in body. It was not until he was again restored to God's favour out of this self-incurred peril, that he went to his army which lay before Rabbath Ammon, and completed the conquest of the royal city of the enemy. The most satisfactory explanation of the situation referred to in this couplet of Psalm is to be gained from 2 Sam 11-12. Psalm 20:1-9 prays for the recovery of the king, who is involved in war with powerful foes; and Psalm 21:1-13 gives thanks for his recovery, and wishes him a victorious issue to the approaching campaign. The "chariots and horses" (Psalm 20:8) are characteristic of the military power of Aram (2-Samuel 10:18, and frequently), and in Psalm 21:4 and Psalm 21:10 we perceive an allusion to 2-Samuel 12:30-31, or at least a remarkable agreement with what is there recorded.