Psalm 20 and 21 should be understood together. Psalm 20 is a national anthem, of sorts, for Israel that gives praise to God for giving Israel victory in battle before battle. Psalm 21 praises God for victory after the completion of the battle. 

Psalm 20 is a Royal Psalm recognizing the King as God’s anointed representative to the people. Careful observation notices the concern for the King in this psalm; a concern that leads ultimately to the Messiah. 

Biblical commentator Willem VanGemeren’s introductory thoughts on this Psalm are most helpful. 

“The congregation actively participates in the royal preparations for war. An enemy has attacked or has shown intent to attack. At stake is the royal glory. But more than that, God’s honor is threatened. The prayer of intercession is for the king and therefore, for the kingdom of God on earth. The prayer takes the form of a blessing by the repetitive use of “may” and is an appropriate prayer for the full establishment of the kingdom of the Messiah of David. 

At the very heart of the petitions and expressions of solidarity is the hope that soon God will grant victory to his ‘anointed’ (vs. 6, = messiah) and that the ‘distress’ will turn into rejoicing. The OT saints looked constantly for the fulfillment of this hope. In Jesus’ coming, we receive further assurance that the kingdom of God is being established. The followers of Jesus may implore the Lord for full establishment of the messianic kingdom, when every knee will bow (v. 8; cf. 1 Cor. 15:25-26; Phil. 2:10), the kingdom is established in his hand (v. 7; cf. 1 Cor. 15:27), and the victory is given by the Father to the Messiah of David (v. 6; cf. 1 Cor. 15:28). In anticipation we may join OT saints in rejoicing, lifting up the banners (v. 5), the assurance of hope (v. 6, ‘Now I know’), and the prayer for Jesus’ establishment of the kingdom (v. 9).” The Expositor’s Bible Commentary, p. 188