“לַיהֹוָה הַיְשׁוּעָה עַל עַמְּךָ בִרְכָתֶךָ סֶּלָה”

“It is incumbent upon the Lord to save, and it is incumbent upon Your people to bless You forever.”

Lots of enemies fought against King David, never had he any prevention from fighting and killing them, by the thousands.
Here, it is all different. In his war against Absalom, the soldiers he’s facing are people of his own.

King David loves the Jewish people, he doesn’t want them defeated, although they chose Absalom. King David wishes the success and salvation of Israel, even though most of the people oppose and chase him.

“Salvation is the Lord’s, bless your people with it.” (A different translation for verse)