1) The example of Jesus is against it. He went to the cross willingly and instructed His followers not to fight for Him (John 18:36). And He calls all of us to take up our own cross and follow in His footsteps (Matthew 16:21-26).
2) The example of the apostles and Christian history is against it. According to tradition, all but one or two of the apostles died as martyrs, and none of them resisted crucifixion or beheading or stoning or drowning. This has been the pattern of church history, to the point that Tertullian famously remarked that the blood of martyrs was the seed of the church. Could you imagine Paul leading an armed revolt against Rome to stop the killing of the followers of Jesus, many of whom were burned alive as torches to entertain the emperor?
3) Jesus explicitly taught against fighting back. In the Garden of Gethsemane, shortly before His crucifixion, He told Peter to put down his sword, explaining that all who lived by the sword would die by the sword (Matthew 26:51-54).
4) The entire testimony of the New Testament is against us violently fighting against our persecutors. Instead, we are called to pray for them (Matthew 5:43-48), we are described as lambs going to the slaughter (Romans 8:35-39; 1 Peter 2:21-23), we are promised persecution (2 Timothy 3:12; John 15:18-20), and only those who suffer with Jesus will reign with Him (Romans 8:16-18). Great is their reward in heaven (Matthew 5:10-12).
5) Jesus told His disciples that if they were persecuted in one city, they should flee to another (Matthew 10:22-25). He could have given military alternatives as well, but He did not.
6) If you do fight back, where do you draw the line? We still have not lived down the bad reputation of the Crusades (even if some of the infamy is exaggerated), and once you get into the military mindset, it’s very hard to know where to draw the line. Perhaps pre-emptive strikes are needed? Perhaps an offensive campaign is called for to offset the next campaign against us? Perhaps our violence will end up being no better than the violence that was used against us?