Dora Bas Rivka Silver O'H
You Win Some, You Lose Some
According to the introductory note, this Psalm was to be accompanied on an instrument called the shushan eidus, or "rose of testimony," which may be an allusion to the Sanhedrin. David composed this Psalm during the war in which his forces, under Yoav's command, subjugated Edom, as described in II Samuel 8.
David says that G-d originally allowed the enemy to make incursions, as if He had abandoned Israel. Clearly He permitted this because He was angry with us, so we pray for forgiveness. G-d made the ground tremble; as the borders of Israel were breached by the enemy, so may they be restored. G-d allowed the nation to taste bitter defeat, but then He allowed us to defeat the enemy, in order to keep His promise to Israel. G-d allowed us to overcome so that His beloved nation could be released from trouble; may He save with His strong right hand and answer our prayers. (This verse is said in the prayer at the end of the Shemoneh Esrei.)
G-d said through prophetic words that grant victory to David and divide the spoils. G-d owns both the land that had been controlled by Saul's son Ishboshes and the land that David had controlled. He owns Edom and Moav like you and I might own personal property (though He doesn't necessarily hold them in high regard and will treat them accordingly). David calls upon the Philistines to subjugate themselves to G-d voluntarily, before they are conquered anyway.
David asks a rhetorical question: Who will help him conquer Edom? (Answer: G-d.) Sometimes G-d lets them lose, but ultimately He grants us victory. David asks for continuing help from G-d against our enemies; G-d's help is better than any human ally! With G-d's help, we are united and invincible.