Mark 13:7-8
7 When you hear of wars and rumors of wars, do not be alarmed; this must take place, but the end is still to come. 8 For nation will rise against nation, and kingdom against kingdom; there will be earthquakes in various places; there will be famines. This is but the beginning of the birth pangs.
Matthew 8:5-10 (Words of Jesus in red.)
5When he entered Capernaum, a centurion came to him, appealing to him 6and saying, "Lord, my servant is lying at home paralyzed, in terrible distress." 7And he said to him, "I will come and cure him." 8The centurion answered, "Lord, I am not worthy to have you come under my roof; but only speak the word, and my servant will be healed. 9For I also am a man under authority, with soldiers under me; and I say to one, 'Go,' and he goes, and to another, 'Come,' and he comes, and to my slave, 'Do this,' and the slave does it." 10When Jesus heard him, he was amazed and said to those who followed him, "Truly I tell you, in no one in Israel have I found such faith.
Many people make the mistake of believing the Bible says, “You shall not kill,” and seek to apply this command to war. However, the Bible actually says, “You shall not murder” (Exodus 20:13). The Hebrew word literally means “the intentional, premeditated killing of another person with malice.” God often ordered the Israelites to go to war with other nations (1 Samuel 15:3; Joshua 4:13). God ordered the death penalty for numerous crimes (Exodus 21:12; 21:15; 22:19; Leviticus 20:11). So, God is not against killing in all circumstances, but rather only murder. War is never a good thing, but sometimes it is a necessary thing. In a world filled with sinful people (Romans 3:10-18), war is inevitable. Sometimes the only way to keep sinful people from doing great harm is by going to war with them.
War is a terrible thing! War is always the result of sin (Romans 3:10-18). In the Old Testament, God ordered the Israelites to: “Take vengeance on the Midianites for the Israelites” (Numbers 31:2). See also Deuteronomy 20:16-17, “However, in the cities of the nations the LORD your God is giving you as an inheritance, do not leave alive anything that breathes. Completely destroy them--the Hittites, Amorites, Canaanites, Perizzites, Hivites and Jebusites--as the LORD your God has commanded you.” Exodus 17:16 proclaims, “He said, "For hands were lifted up to the throne of the LORD. The LORD will be at war against the Amalekites from generation to generation." Also, 1 Samuel 15:18, “Go and completely destroy those wicked people, the Amalekites; make war on them until you have wiped them out.” So, obviously God is not against all war. Jesus is always in perfect agreement with the Father (John 10:30), so we cannot argue that war was only God’s will in the Old Testament. God does not change (Malachi 3:6; James 1:17).