Picking your battles, rather than trying to fight and win every fight that comes your way, is a good piece of advice. However, who should pick which battles to fight? The Old Testament book of Judges is a record of the consequences of Israel’s failure to completely conquer the promised land, a battle God gave them to fight and win. Judges also shows more generally what happens when anyone picks the wrong battles to fight: they end up with less than what God intended for them.
For example, Judges 1:19 tells us: “And the LORD was with Judah, and he took possession of the hill country, but he could not drive out the inhabitants of the plain because they had chariots of iron.”
This verse, especially the second part, is written from a human perspective. God did not tell Israel to conquer all of the promised land, except for the plains. God is not afraid of chariots. He told them to conquer all of it, but Israel thought that chariots couldn’t be defeated, so they decided not to fight in the plain. By using their own judgement and preferring to fight in hills or forests where chariots were less effective, they failed to fully receive what God had promised them.
An application to us is that our inheritance is Christ’s righteousness, and Jesus tells us “Blessed are those who hunger and thirst for righteousness, for they shall be satisfied.” Jesus is telling us that righteousness is like food and drink. We can never have enough because no matter how much we eat and drink, our hunger and thirst soon return. We are all in different places, at different levels of knowledge and maturity, but in Christ we are all on the same path and have an appetite only He can satisfy.
To be satisfied, we cannot settle for what we have already accomplished, the hill country we have already taken. To be satisfied, we must fight the chariots in the plains if that is what God wants us to do, so that we learn to rely on His strength. To be satisfied, we move from only fighting battles we choose based on our own wisdom and ability to choosing the battles we will win in His strength.
Sometimes life is hard on purpose, so that God alone may be glorified in victory, and also so that we may grow in our faith in His strength. When we let Him pick our battles, we learn that His righteousness is the only thing that will satisfy us. Nothing less will do.
 Matthew 5:6