(From a friend's post on Facebook.)
It's different circumstances.
When God called Israel by Moses, He called them to be a "kingdom of priests and a holy nation", see Exodus 19, especially v. 6. Peter reiterated this to the early Christians, per 1st Peter 2, around v. 9.
But the Israelites in general could not handle it, so the Levites took on the burden of the preparatory priesthood of external ordinances, and were supposed to teach the ways of God to the rest. They were mediators between the people and God.
Israel was supposed to teach the ways of God to the world around them, but the people around them only knew war. So Israel had to go to war.
Israel was supposed to start by suing for peace. In some cases, the countries they visited wouldn't even give them a chance to do so. In the case of the Philistines and Goliath, the Philistines were already causing a scene.
Israel was an example of God making His power easy to see.
Getting us to change our ways, to turn ourselves towards God, is what God wants of us. External displays of power don't really help much to that end.
In Matthew 5: 41, Christians are now somewhat on our own, both before God and before the world. Also, we are no longer to depend on a Levitical priesthood to mediate between us and God. Jesus Himself is our mediator. And we are supposed to teach by example.
It has greater power to convert the soul to serve rather than to fight, and, because of the rules in the world at the time, the soldiers did have the right to demand peasants and Jews carry their equipment for them. By going with them twice the required distance, they had more opportunity to preach the Gospel while avoiding the sort of confrontation that would hasten the coming persecutions.
"Not cause a scene" is not what God said, not what He has in mind.
One of the things that is most difficult in understanding the Sermon on the Mount is that these aren't easy rules like 1000 steps on the Sabbath. These are commandments that require the believer to listen to the Holy Spirit in order to obey. Even now, we sometimes have to slay giants with small stones instead of swords, but the giants we are called to slay first are our own pride and lusts -- again, with small means that actually works, not with big action that seems to satisfy but doesn't.
(Sorry for the sermons, I need them for myself.)