"Do not be overcome by evil, but overcome evil with good" (Romans 12:21).
Our human tendency is to repay evil with evil. It's easy to like people that treat us well; but, what about those that want to "throw us under the bus?" The Bible directs us to always do what is good, regardless of the circumstances, and never do harm to our neighbors, even when we think they deserve it. If we do the same wrongdoing they have done, then we are only making ourselves just like them.
The question then becomes: If I always do what is good and the other person seems to always want to jeopardize me, am I always going to be at a loss? This is exactly what happened to King Jehoshaphat. Three nations united to destroy him, even though years before the ancestors of Judah, in obedience to God, preserved those same enemies. These people should have been thankful, but instead, they opted to declare war to those that had done good to them. What a flop for the king and his people. The good that they had done was being repayed with evil - what to do now?
Faced with a very difficult decision in which Judah would apparently suffer a loss, the king and all the people cried out to God for help and handed over the situation to Him, trusting in His doing. God sent them a word of encouragement, and when they left for war, they took with them a very unusual weapon: worship to God. When they started worshiping, God himself took action towards the enemies of the people.
When we use the weapons of dependence, trust, obedience, and worship to God instead of weapons of destruction which most use, God will go in front of us. Even when we do good but continue to receive evil, let's not be discouraged nor give up. Hand over everything to God and let Him resolve it for you.
Against evil, the most powerful weapon is dependence on God.