Spiritual Warfare with the World

What does the Bible mean when it suggests that we are in a battle against the world? It isn't people, nations, or governments. Rather, it is the systems and structures that are in opposition to the kingdom of God. This battle is real and for us to engage effectively in it we need to be have a right definition and a correct posture. In the same way, we need to know where we've been affected and how to reclaim that ground.