For we do not wrestle against flesh and blood, but against the rulers, against the authorities, against the cosmic powers over this present darkness, against the spiritual forces of evil in the heavenly places. (Ephesians 6:12)
To fight against the darkness we come to God with our hearts and minds prepared by fasting and praying, and we present our request of winning against the evil and overcoming the spiritual warfare. One of the most powerful weapons is fasting prayers.
By fasting, our carnal impulses are forced to relinquish control. It compels us to accept that what we fight against is not external. The true enemy is spiritual and the battlefield is our heart. When we fast, we position ourselves for God to move.
For though we walk in the flesh, we are not waging war according to the flesh. For the weapons of our warfare are not of the flesh but have divine power to destroy strongholds. We destroy arguments and every lofty opinion raised against the knowledge of God, and take every thought captive to obey Christ, being ready to punish every disobedience, when your obedience is complete. (2 Corinthians 10:3-6)
In the days of Mordecai and Esther, all the Jews fasted and prayed together (Esther 4:16 ) in order to seek victory in battle against their enemies. God is pleased when we fast and His favor rests on our side.
We fast and pray for God to intervene and give us victory against the enemy. Our victory comes only from Him.