turn the other cheek or buy a sword

Reinhold Niebuhr pointed out two schools of Christian thought on the question of war: one was trying to avoid war at all cost by holding to the ideals of love and the character of the Christ, while the other one held war as the an ultimate response to tyranny which would be worse that the war itself. He argues that some modern versions of Christianity do not realize that human evil is a problem that needs a radical solution and that every Christian has the responsibility to take action as an instrument of God's justice. If the result of the war had disastrous implications/consequences for the whole world, how could Christians have avoided helping other nations?