Grace out your enemies.
"Beloved, never avenge yourselves, but leave it to the wrath of God, for it is written, 'Vengeance is mine, I will repay, says the Lord.' To the contrary, 'if your enemy is hungry, feed him; if he is thirsty, give him something to drink; for by so doing you will heap burning coals on his head.' Do not be overcome by evil, but overcome evil with good." - Romans 12:19-21 ESV
Treat them like friends and brothers. But, That sounds so naive. Yes, Maybe. If there is no God and if love is powerless.
If we are sentimental friends and brothers it is also dangerous. If our obligation extends beyond love we stop listening to God and become deaf to His initiations.
"Owe no one anything, except to love each other, for the one who loves another has fulfilled the law. For the commandments, 'You shall not commit adultery, You shall not murder, You shall not steal, You shall not covet,' and any other commandment, are summed up in this word: 'You shall love your neighbor as yourself.' Love does no wrong to a neighbor; therefore love is the fulfilling of the law." - Romans 13:8-10 ESV
Nehemiah had a great approach to warfare.
"From that day on, half of my servants worked on construction, and half held the spears, shields, bows, and coats of mail. And the leaders stood behind the whole house of Judah, who were building on the wall. Those who carried burdens were loaded in such a way that each labored on the work with one hand and held his weapon with the other." - Nehemiah 4:16-17 ESV
"Sanballat and Geshem sent to me, saying, 'Come and let us meet together at Hakkephirim in the plain of Ono.'
But they intended to do me harm. And I sent messengers to them, saying, 'I am doing a great work and I cannot come down. Why should the work stop while I leave it and come down to you?'
And they sent to me four times in this way, and I answered them in the same manner." - Nehemiah 6:2-4 ESV
Our wall building is grace oriented. Our sword is grace. It has been beaten into a plowshare and a pruning hook (see Isaiah 2:4). Our laboring tools are the laboring tools of grace. Rest and the spreading of the message of rest. We labor to share the Gospel of Grace.
The difference between a friend and an enemy is the fellowship. We can give to an enemy the same way we give to a friend but we can not invite them into our walls the same way we would a friend.
Judas was given all that every other disciple was given but he never heard the words Jesus said at the last supper after he was gone. He never sang as they went... he never prayed in the garden of Gethsemane.
"Can a man take fire in his bosom, and his clothes not be burned?" - Proverbs 6:27 KJV
The context of this verse is adultery but the principle stands. Intimacy is not for everyone. We love everyone but we are not intimate with everyone.
"I am acting with great boldness toward you; I have great pride in you; I am filled with comfort. In all our affliction, I am overflowing with joy. For even when we came into Macedonia, our bodies had no rest, but we were afflicted at every turn -fighting without and fear within. But God, who comforts the downcast, comforted us by the coming of Titus, and not only by his coming but also by the comfort with which he was comforted by you, as he told us of your longing, your mourning, your zeal for me, so that I rejoiced still more." - 2 Corinthians 7:4-7 ESV
When there is fellowship there is refreshment.
The great thing about grace, one of a long list of great things, is that you may find that people that were once your enemies will become your friends. God will convert them and make them friends of His and they will become friends of yours.