In his commentary "Be Resolute," Warren W. Wiersbe speaks about spiritual warfare. He knew what has happened to many of us today when we mention the subject and get mocked. Know that moderate, well-educated Bible teachers believe that such a thing exists. If you work overseas in a poorer country, you will confront more demonic activity than you could ever expect. 

Daniel’s conversation with the angel reveals to us the important fact that there is an “invisible war” going on in the heavenlies between the forces of evil and the forces of God. For three weeks, Daniel had been praying for wisdom to understand the visions he had already seen, but the answer to that prayer was delayed. Why would the Lord not immediately answer the petitions of His beloved prophet? Because “the prince of the kingdom of Persia”—an evil angel—had attacked the angel bearing the answer, probably Gabriel. This evil angel was assigned to see to it that the king of Persia did what Satan wanted him to do. Michael, the archangel assigned to minister to Israel (Dan. 12:1; Rev. 12:7; Jude 9), assisted Gabriel and together they won the battle.

Well-meaning people may scoff at the idea of demonic forces and good and evil angels, and they may caricature Satan, but the fact remains that this is biblical theology. When Lucifer rebelled against God and was judged, some of the angels fell with him and became the demonic evil angels that oppose Christ and obey Satan (Isa. 14:12–15; Rev. 12:7–12; Matt. 25:41). According to Ephesians 6:10–18, Satan has a well-organized army of evil spirits that obey his every command. Through His sacrificial work on the cross, Christ defeated Satan and his army (1:20–23; Col. 2:15; John 12:31; Rev. 12:11), and we can claim that victory by faith. The believer’s responsibility is to put on the whole armor of God by faith and use the Word of God and believing prayer to oppose and defeat the wicked one.

It appears that there are specific evil angels assigned to various nations; some students of angelology call them “territorial spirits.” That’s why Paul told the Ephesian believers that the Christian’s battle was not against flesh and blood but against demonic forces in the heavenlies that oppose the holy angels who always do God’s will. The problems that the Jewish remnant were having in Jerusalem at that time weren’t being caused by the local officials but by Satan’s evil powers using those officials. Christians are never to worship angels (Col. 2:18–19; Rev. 19:10; 22:8–9) or pray to angels, for our worship and prayer belong to God alone. But when we pray, God directs the armies of heaven to fight on our behalf, even though we may know nothing about the battles that are being waged in this invisible war. (See 2 Kings 6:17.)