Objection 1. It would seem that peace is a virtue. For nothing is a matter of precept, unless it be an act of virtue. But there are precepts about keeping peace, for example: "Have peace among you" (Mark 9:49). Therefore peace is a virtue.
Objection 2. Further, we do not merit except by acts of virtue. Now it is meritorious to keep peace, according to Matthew 5:9: "Blessed are the peacemakers, for they shall be called the children of God." Therefore peace is a virtue.
Objection 3. Further, vices are opposed to virtues. But dissensions, which are contrary to peace, are numbered among the vices (Galatians 5:20). Therefore peace is a virtue.
On the contrary, Virtue is not the last end, but the way thereto. But peace is the last end, in a sense, as Augustine says (De Civ. Dei xix, 11). Therefore peace is not a virtue.
I answer that, As stated above (Question 28, Article 4), when a number of acts all proceeding uniformly from an agent, follow one from the other, they all arise from the same virtue, nor do they each have a virtue from which they proceed, as may be seen in corporeal things. For, though fire by heating, both liquefies and rarefies, there are not two powers in fire, one of liquefaction, the other of rarefaction: and fire produces all such actions by its own power of calefaction.
Since then charity causes peace precisely because it is love of God and of our neighbor, as shown above (Article 3), there is no other virtue except charity whose proper act is peace, as we have also said in reference to joy (28, 4).
Reply to Objection 1. We are commanded to keep peace because it is an act of charity; and for this reason too it is a meritorious act. Hence it is placed among the beatitudes, which are acts of perfect virtue, as stated above (I-II, 69, 1,3). It is also numbered among the fruits, in so far as it is a final good, having spiritual sweetness.
This suffices for the Reply to the Second Objection.
Reply to Objection 3. Several vices are opposed to one virtue in respect of its various acts: so that not only is hatred opposed to charity, in respect of its act which is love, but also sloth and envy, in respect of joy, and dissension in respect of peace.
Source: Summa Theologica (New Advent)