What does it mean that the flesh wars against the Spirit?

Les explains that the old nature actually declares war on the new nature, so the new nature must declare war on the old nature, making it a constant battle. The battle lines are defined from the moment of salvation until death or the Rapture, so until then believers are in constant warfare. The old Adam, or flesh, lusts and wars against the Spirit, which implies the new nature, and the Holy Spirit wars against the flesh, or old Adam. These two forces in every believer are contrary, at total odds, and cannot agree on anything. The new nature has a whole new set of values in compliance with the righteousness, holiness, and laws of God, whereas the old nature, whose God is Satan and the god of this world, is bent in rebellion and unbelief. They are two totally opposite forces and war constantly, so you cannot do the things you would.