Why is their war ?

And war broke out in heaven: Michael and his angels fought with the dragon; and the dragon and his angels fought.
Revelation 12:7

Recommended Reading
Daniel 10:1-21


Many have asked, "Why is there war?" It is simple: Earth has war because there is war in heaven. Ever since God's brightest angel, Lucifer, rebelled against his Creator-God and sought to unseat Him from His throne in heaven (Isaiah 14:12-14), conflict has been ongoing. Just as Satan opposed righteousness and justice in heaven, so he has opposed it on earth. He stirs up greed, envy, and ambition in human hearts, causing others to defend themselves against invasions and attacks. Even sincere Christians differ over how to respond to sinful aggression. The irony is that God Himself will use history's greatest battle to put an end to satanic and human rebellion on earth (Revelation 19:11-21).

As children of God, we have peace in our hearts, and when the "Prince of Peace" returns to rule and reign He will put an end to all war. Until that time comes, we need to keep in the Word, fully armed with the Sword of the Spirit, ready for spiritual battle and Christ's ultimate return.

If you are to have peace with God, there must be war with Satan.
Charles H. Spurgeon