Psalm 35:1 opens: “Contend, O Lord, with those who contend with me; fight against those who fight with me.” The key principle taught here is: let GOD do your fighting for you.
As God’s people, we must not give in to earthly manipulations and fleshly efforts to “get back” at people who have harmed us, or who oppose us. Instead, let GOD be the One who works on your behalf. This is the “meekness” which Jesus commends in the Sermon on the Mount: not taking things into own hands, but entrusting ourselves to God instead (Matthew 5:5)
— David, who wrote this Psalm, lived out an example of that; he told Saul, who was trying to kill him, “May God judge between you and me … but my hand shall not be against you.” (I Samuel 24:12) David spoke those words just after he could have killed Saul in a cave, but did not. He allowed God to take care of Saul in His own time, and His own way — which He subsequently did.
— Jehoshaphat is another example: his troops didn’t go out to engage the enemy army, but to sing to God, and HE fought for them in a miraculous way. (II Chronicles 20)
Thus we learn that when we allow God to fight our battles for us, HE does it FAR better than we could! Some of us today need to determine to lay down our puny “weapons” of attack or revenge, and entrust a particular battle to the Lord instead.
Now, “letting God fight for you” doesn’t always mean that you stand by and do nothing. Sometimes it does, but other times it means that although we do what we must, our trust is not in our own efforts but in the Lord. When David’s enemies came against him, he did defend himself. The English King Henry V famously claimed that “God fought for us” at Agincourt — but that did not mean he did not wield a sword in battle! But in both cases although they themselves fought, their trust was that God was helping them do what they could not have done without Him. That is what we should do too. There are times when we should “be still and know that (He is) God”, refuse to defend ourselves, and let Him completely take care of a matter on our behalf. Other times, we are compelled to strive. In either case, the most important thing is that your trust is in God, and not in your own efforts. Make your prayer to God like David’s: “Fight against those who fight with me”!