I believe that to correctly understand Jesus' words, we must understand the context of his life, and who he was talking to. He was not speaking in a vacuum. Everything He said was in the context of the Perfect Law of God.
Jesus Himself said he did not come to destroy the Law.
John says the definition of sin remains the same it was under the OT.
And Paul said that even in NT times, he could not have known sin, except for the Law.
And that the Law is a schoolmaster, to bring us to Christ.
The OT moral code was given to reveal God's moral standard.
God has not changed. His nature, His likes and dislikes, have all remained the same.
The Law was perfect as a moral standard.
Therefore, Jesus never could introduce to us a higher standard.
Everything Jesus said--even in the Sermon on the Mount--never contradicted or replaced the moral code of the Law, with a higher, more perfect moral code. In fact, everything Jesus said in the Sermon on the Mount is actually found in the OT! Even "love your Enemies!"
We cannot obtain righteousness by obeying the Law.
That's why Jesus came; to completely wash away sin, which the Law could never do.
But the definition of sin remains the same.
Therefore, Jesus' words must be understood in the context of the OT moral code.
When God said, "Thou shalt not murder," He was NOT prohibiting war.
Instead, He commanded war!
He was not prohibiting the death penalty, either. After all, look how often God commanded His people carry out the death penalty.
Instead, "Thou shalt not murder" was talking about premeditated murder against your personal neighbor, or your fellow countryman.
When Jesus said "My kingdom is not of this world, if it were of this world, then would my servants fight" He finished his statement with a qualifier: "...that I should not be delivered to you."
In other words, every kingdom has to fight and make war. When we operate as citizens of this earth, we are part of earthly kingdoms, and have earthly citizenship.
When we operate as citizens of the kingdom of heaven, we must operate as ambassadors of the heavenly kingdom.
We hold dual citizenship. Paul used his Roman citizenship to avoid a beating, and on another occasion, he used it to shame those who had just beaten him and thrown him in jail.
The way I see it, God does not consider it sin, to fight in war, in defense of a righteous cause. But when in a situation that it's a personal wrong done to us, we are not to rise up and repay personal hurt. Defending others, such as our families... that's commanded in Scripture. God hates it when a strong person sits by and does not defend the weak.
I grew up in a very pacifist setting. Mennonites and Amish do not believe in even defending their families. But this warps people. I have seen, within that culture, where no one would stand up in defense of a handicapped person, when they were abused... because they were trained not to think in terms of defending others.