David was about to set out for war. Because he knew he could never achieve victory on his own, he first conducted morning prayer at the tabernacle, leading the people to pray for the upcoming battles. The psalm we have before us today is that prayer.
The first thing David does in the prayer is lead the people to pronounce a benediction - on himself! They sing, "May the Lord answer you in the day of trouble; may the Name of the God of Jacob defend you; may He send you help from the sanctuary, and strengthen you out of Zion; may He remember all your offerings, and accept your burnt sacrifices!" That's just what David was going to need when he got out onto the battlefield - he was going to need "help in the day of trouble" - defense at the hand of God, help from the God Who revealed Himself in the sanctuary, and strength from the God Who dwelt in Zion.
So all the people are led to pray for their field commander. They pray, "May He grant you according to your heart's desire, and fulfill all your purpose."
He follows this by leading them to express what they should feel as they hear of God's blessing on the army. They repeat, "We will rejoice in your salvation, and in the Name of our God we will set up our banners! May the Lord fulfill all your petitions!"
So, because all Israel would be blessed by a successful war, David leads them in prayer for himself and the army. But he also knows that Israel is God's special people, so he he then leads them in an expression of confidence. They sing, "Now I know that the Lord saves His anointed; He will answer him from His holy heaven with the saving strength of His right hand." He knows he is the Lord's anointed, that he is therefore acting on behalf of the Lord, and that he will therefore experience the Lord's salvation in battle!
Of course David also wants the people to realize that there's no use of them trusting in military power for victory, so he leads them to sing, "Some trust in chariots, and some in horses, but we will remember the name of the Lord our God!" The result of such trust will be the enemy's defeat. The people sing, "They (the enemy) have bowed down and fallen, but we have risen and stand upright!"
Then he leads them in one final prayer, this time for themselves and their nation. He leads them to sing, "Save, Lord! May the King answer us when we call." He wants them to pray that he, David, will be their defender, that he will this time, as always, be able to answer their call for defense.
A war psalm like this is directly applicable to our own lives as God's people. We too are involved in an intense warfare. Our war, too, cannot be won by our own strength or wisdom. Since Jesus Christ is our Warrior-King, we too should sing,"May the King answer us when we call!" May Jesus Christ defend us against the world, the flesh, and the Devil. Amen!