Actually, there is only one passsage that I found that appears (at first glance) to be in strong favor against pacificism. I say this because I want to be open and honest with everyone about Scripture with people here and let them know I am not out to just look at one side of the Bible.
That passage is
Matthew 8:5-13.
Jesus appears to favor of the Centurion's example in relation to his great faith that Jesus will be able to heal the one he asks to be healed.
Okay, you have to understand that this Centurion man is a Gentile. At this time: Gentiles were generally categorized as unbelievers and Jesus was preaching to the house of Israel first before spreading that gospel message out to the Gentiles. For do you remember the woman who was a Gentile and called to Jesus and she related about how even the dogs eat the crumbs from the table? This is important to understand because Jesus' gospel message and teachings were going out to the house of Israel first. For in the Centurion's world, God was using him as a minister of justice. For Romans 13 says that all nations that are set into power by God (Which are all unbelieving nations: Which pictures Gentiles) are God's ministers of executing justice. This can be seen with the King of Assyria in
Isaiah 10:5-12 NLT (For God uses the King of Assyria (who is an unbelieving king of an unbelieving nation) to punish Israel for their sins). In other words, at the time Jesus talked to the Centurion: the teachings that Jesus gave about loving your enemies in Matthew chapter 5 was to the house of Israel and it was not for the Gentiles yet. For the time of including the Gentiles did not happen yet until Jesus gave the great commission after His resurrection. For Jesus said,
"I have other sheep, which are not of this fold; I must bring them also,..." (
John 10:16). Jesus also said,
"Go ye therefore, and teach all nations,..." (Matthew 28:19).
My point being is that, first, Jesus is not out to teach this Gentile yet (His ways until after His resurrection); Second, Jesus is not against authority within armies because Jesus is one with the Father, (i.e. second person of the Godhead or the Trinity) who executes justice thru His unbelieving ministers of justice or unbelieving nations; For there it is very likely that God used this Centurion's army (that were unbelievers / Gentiles) in the past for his purposes. So Jesus' approval of his authority in relation to his faith shows that God was still continuing to work His authority in unbelieving (Gentile) nations to execute justice for His purposes and plan.
Side Note: Now, do not misunderstand me, God did save Gentiles in the Old Testament. The story of Jonah and others tells us this. But Jesus' gospel was to the Jew first, and then to the Greek.