We are taught in Romans 12:18 (KJV), "If it be possible, as much as lieth in you, live peaceably with all men."
"IF IT BE POSSIBLE!!!"
When Paul wrote this, it was with the understanding the SOME persons are so bad that we just can NOT live peaceably with them. We are to love our enemies, but sometimes they need an 'attitude adjustment' so you can get their attention.
It is the nature of the fleshly beast to war- it is also of interesting note that some of the greatest armies in the world were christian beacause they knew no fear- there is no place in the bible that condems a man for being a soldier or going to war- but it does warn against personal corruption- sometimes it takes a brave and godly people to stop a tyrant- it has often been said that Hitler would have been easy to stop when he first took Poland but giving him time and opportunity cost millions of lives.