The exhortation to Praise the LORD in Psalm 149 begins with: Sing to the LORD a new song. Singing to the LORDa new song appears in at least five other psalms. The new song praises Him for His attributes:
Sing to Him a new song; play skillfully with a shout of joy, for the word of the LORD is upright; and all His work is done in faithfulness.
He loves righteousness and justice; the earth is full of the lovingkindness of the LORD (Ps. 33:3-5).
In many instances, the new song praises the LORD for His deliverance from trouble and from enemy nations. For example:
O Sing to the LORD a new song, for He has done wonderful things, His right hand and His holy arm have gained the victory for Him (Ps. 98:1).
Indeed, here in Psalm 149:5-9, the song accompanies a passionate plea for victory over the enemies of God and of His people:
Let the godly ones exult in glory; let them sing for joy on their beds.
Let the high praises of God be in their mouth, and a two-edged sword in their hand,
To execute vengeance on the nations, and punishment on the peoples;
To bind their kings with chains, and their nobles with fetters of iron;
To execute on them the judgment written; this is an honor for all His godly ones. Praise the LORD!
It may be hard for us to put this together, but it was not a new thing for Israel. In 2 Chronicles 20 we read of Jehoshaphat, King of Judah, and his army up against huge odds. God sent His prophet to tell the king not to fear and to go and face the enemy; but also that he need not fight, but stand and see the salvation of the LORD. So the next morning the king put those who sang to the LORD in front of the army as they went out. After a great victory, it says: every man of Judah returned to Jerusalem with joy, for the LORD had made them to rejoice over their enemies (2 Chron. 20:27). Sounds a lot like Psalms 98, 144, and 149.
Perhaps the lesson for us is that when the LORD gives us a hard task, to go after it with a song of praise, and then to be ready with a new song to celebrate His deliverance.