Have you noticed how many war stories are in the Bible? If you want to conduct spiritual warfare you will find good lessons hidden in those often gruesome stories. Many people will skip over those chapters, or the books of Joshua and Judges because they are too bloody and violent.
But wait. If God wanted to give His saints secret instruction for fighting against spiritual principalities and powers that we cannot see, where would there be a better place to hide them than in stories that the uncaring person would avoid? Or, in such a place that only the Holy Spirit could interpret for you?
That's right. Those war stories in the Old Testament are illustrated lessons for us on how to conduct spiritual warfare.
Why, exactly, would Christians want to know or exercise warfare?
For the same reason that you want to learn effective prayer skills. When people are in bondage to lies and evil spirits, we need to know how to come against them, cast them out and bring deliverance. We can only do that when we are mighty in God's Spirit and rightly use the Word of God, which is our weapon. We don't go out seeking war of this kind, but it often springs on us.
Spiritual warfare is very much tied to effective prayer. That is why we refer to people who are mature and adept at this as prayer warriors.
By S. D. Gordon, Quiet Talks on Prayer:
"Prayer from God's side is communication between Himself and His allies in the enemy's country." (page 37)
"Prayer concerns three, not two, but three: God to whom we pray, the man on the contested earth who prays, and the Evil One against whom we pray. And the purpose of the prayer is not to persuade or influence God, but to join forces with Him against the enemy." (page 120)
Satan has assigned his minions of evil spirits to work especially in certain parts of the world, in certain areas of influence, such as at all levels of government, in the entertainment world, and even in churches. These would be principalities.
A great many evil spirits operate particularly on the personal thoughts and feelings of Christians as well as unbelievers. They inspire thoughts and plans for crimes, or things that appear as pride, revenge, and self-gratification. Sometimes they are subtle and sometimes not so subtle.
The spiritual rulers or forces of Ephesians 6, would be those that encourage and start up superstitions, fortunetelling, and false doctrines that lead people astray, and eventually into realms of darkness and bondage.
Ephesians 6:12 says, "For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms." That is why we are to put on the full armour of God and wage spiritual warfare against those rulers, authorities and powers of darkness.
So yes, we need to study the war stories in the Bible for lessons in more effective spiritual warfare - prayer. Though this kind of praying is available to all believers, it is not for the faint of heart. It takes strong faith and tremendous persistence. Some are fought in a moment or a few minutes of prayer, but others take days, weeks, or even years!
Try reading about the battles the Israelites had in the Old Testament as parables, with your eyes open to spot the principles for effective prayer against evil forces and deeds. You will be astonished at what you learn.