How has war been justified within Christianity, with respect to the New Testament and preferably the Gospels? Just really a question of curiosity on how the faith has, at times, turned down the wrong path of aggression and violence, specifically wondering what passages of the New Testament were used as justification for such action.
On war: how has it been justified within Christianity, with respect to the New Testament, preferably the Gospels?

Many Christians believe they have the right to defend themselves against an open aggressor, even to go to war if necessary. See: Here.

We can notice in the Gospels that Our Lord never once condemned a soldier for being a soldier. In fact he once actually healed a Centurion’s servant.


  The Faith of a Centurion
  
  5 When he had entered Capernaum, a centurion came forward to him, appealing to him, 6 “Lord, my servant is lying paralyzed at home, suffering terribly.” 7 And he said to him, “I will come and heal him.” 8 But the centurion replied, “Lord, I am not worthy to have you come under my roof, but only say the word, and my servant will be healed. 9 For I too 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 servant, ‘Do this,’ and he does it.” 10 When Jesus heard this, he marveled and said to those who followed him, “Truly, I tell you, with no one in Israel have I found such faith. - Matthew 8:5-10


If Jesus thought that war carried on by soldiers was unjustified; why then did he heal the centurion’s servant?

Romans tells us we should be subject to civil authorities at all times.


  Submission to the Authorities
  
  13 Let every person be subject to the governing authorities. For there is no authority except from God, and those that exist have been instituted by God. 2 Therefore whoever resists the authorities resists what God has appointed, and those who resist will incur judgment. 3 For rulers are not a terror to good conduct, but to bad. Would you have no fear of the one who is in authority? Then do what is good, and you will receive his approval, 4 for he is God's servant for your good. But if you do wrong, be afraid, for he does not bear the sword in vain. For he is the servant of God, an avenger who carries out God's wrath on the wrongdoer. 5 Therefore one must be in subjection, not only to avoid God's wrath but also for the sake of conscience. Romans 13:1-5


Let us not forget that St. Michael the Archangel fought the evil angels in the Book of Revelation.

The Book of Revelation demonstrates that the Angels fought Satan and his legions. Now if the Angels can fight against evil, it stands to reason man can fight in order to protect himself from being killed, pillaged and women from being raped!


  Satan Thrown Down to Earth
  
  7 Now war arose in heaven, Michael and his angels fighting against the dragon. And the dragon and his angels fought back. - Revelation 12:7


Remember too that peace is to be sought out when possible. Let us recall the words of Jesus himself in the Gospel of St. Luke.


  31 Or what king, going out to encounter another king in war, will not sit down first and deliberate whether he is able with ten thousand to meet him who comes against him with twenty thousand? 32 And if not, while the other is yet a great way off, he sends a delegation and asks for terms of peace. - Luke 14:31-32


Never once did Jesus openly condemn being a soldier or making war on an enemy.

I really do not see how it is not possible to not fight forces of evil. Hitlers and Stalins must the be defeated. Surely others will arise and we must be ready.