Psalm 89 is a lament over the broken Davidic covenant. The writer is grieving the apparent failure of God's promises to David, specifically the promise of an enduring kingdom. Verse 43, in that context, is a powerful accusation against God. Let's break it down:

"Yes, you turn back the edge of his sword..." This speaks to a weakening of the king's military power. The "edge of his sword" represents the king's ability to defeat his enemies. Turning it back means making it ineffective, blunting it. The king is no longer victorious.

"...and haven't supported him in battle." This is a direct accusation that God, who had previously promised to be the king's ally and protector, has abandoned him in warfare. He isn't providing the necessary strength, strategy, or victory. God is not fulfilling his role as the king's divine warrior.

In essence, the verse is saying:

"You, God, have not only allowed our king's military strength to diminish (his sword's edge dulled), but you have actively withheld your support in battle, leaving him vulnerable and defeated."

The verse highlights the crisis of faith being expressed in Psalm 89. It questions God's faithfulness to the Davidic covenant in light of the king's present suffering and military defeats. The psalmist is challenging God to remember his promises and act on behalf of his chosen king. The psalmist is not necessarily saying God literally physically dulls the sword, but rather that God is the source of the king's power and victory.

The overall tone is one of bitterness, disappointment, and desperate pleading. It's a lament born from a perceived betrayal of divine promises.