Michael and the angels (in white) fighting against Satan and his angels Painting by Peter Breugel the Elder, Royal Museums of Fine Arts, Brussels
The Origin of the Warfare: Rebellion in Heaven
In Isaiah 14:12-15 we are given a glimpse of the rebellion in heaven by one of God's created angels who ministered in God's presence.
How you have fallen from heaven, morning star, son of the dawn! You have been cast down to the earth, you who once laid low the nations! You said in your heart, "I will ascend to the heavens; I will raise my throne above the stars of God; I will sit enthroned on the mount of assembly, on the utmost heights of Mount Zaphon. I will ascend above the tops of the clouds; I will make myself like the Most High." But you are brought down to the realm of the dead, to the depths of the pit.
This beautiful angelic being attempted a take-over bid for God's throne and was banished to earth with a third of the angels who joined his rebellion.
In Ezekiel 28:14-17 we get another glimpse of what happened.
You were anointed as a guardian cherub, for so I ordained you. You were on the holy mount of God; you walked among the fiery stones. You were blameless in your ways from the day you were created till wickedness was found in you. Through your widespread trade you were filled with violence, and you sinned. So I drove you in disgrace from the mount of God, and I expelled you, guardian cherub, from among the fiery stones. Your heart became proud on account of your beauty, and you corrupted your wisdom because of your splendour. So I threw you to the earth; I made a spectacle of you before kings.