Why does it say "Yiten Lecha ki'Lvavecha"?
Radak: He should put your enemies under you, like is in your heart.
Radak citing Shocher Tov: Since David was complete in front of his Creator, and prayed to him with a full heart, they said so. They would not say so to a regular person, lest his heart intends to steal, sin or do something improper. To him, they say "Hashem should fulfill all your requests 1 " (verse 6).
Malbim: The troops who went to help David said so when they heard that he was already saved. This was David's heart's desire!
Surely, one who had sin in his heart, he would not request it from Hashem! (PF)
What is "Atzascha"?
Radak: This is like "Ki v'Sachbulos Ta'aseh Lecha Milchamah" (Mishlei 24:6) - strategy and strength in war.
Malbim (5-6): This was David's counsel, that when we raise the flag of war, it will be in the name of Hashem. He will be our head general in front of the troops; it will be called on His name, and we will follow His flag.
