Objection 1. It would seem that peace is the same as concord. For Augustine says (De Civ. Dei xix, 13): "Peace among men is well ordered concord." Now we are speaking here of no other peace than that of men. Therefore peace is the same as concord.
Objection 2. Further, concord is union of wills. Now the nature of peace consists in such like union, for Dionysius says (Div. Nom. xi) that peace unites all, and makes them of one mind. Therefore peace is the same as concord.
Objection 3. Further, things whose opposites are identical are themselves identical. Now the one same thing is opposed to concord and peace, viz. dissension; hence it is written (1 Corinthians 16:33): "God is not the God of dissension but of peace." Therefore peace is the same as concord.
On the contrary, There can be concord in evil between wicked men. But "there is no peace to the wicked" (Isaiah 48:22). Therefore peace is not the same as concord.
I answer that, Peace includes concord and adds something thereto. Hence wherever peace is, there is concord, but there is not peace, wherever there is concord, if we give peace its proper meaning.
For concord, properly speaking, is between one man and another, in so far as the wills of various hearts agree together in consenting to the same thing. Now the heart of one man may happen to tend to diverse things, and this in two ways. First, in respect of the diverse appetitive powers: thus the sensitive appetite tends sometimes to that which is opposed to the rational appetite, according to Galatians 5:17: "The flesh lusteth against the spirit." Secondly, in so far as one and the same appetitive power tends to diverse objects of appetite, which it cannot obtain all at the same time: so that there must needs be a clashing of the movements of the appetite. Now the union of such movements is essential to peace, because man's heart is not at peace, so long as he has not what he wants, or if, having what he wants, there still remains something for him to want, and which he cannot have at the same time. On the other hand this union is not essential to concord: wherefore concord denotes union of appetites among various persons, while peace denotes, in addition to this union, the union of the appetites even in one man.
Reply to Objection 1. Augustine is speaking there of that peace which is between one man and another, and he says that this peace is concord, not indeed any kind of concord, but that which is well ordered, through one man agreeing with another in respect of something befitting to both of them. For if one man concord with another, not of his own accord, but through being forced, as it were, by the fear of some evil that besets him, such concord is not really peace, because the order of each concordant is not observed, but is disturbed by some fear-inspiring cause. For this reason he premises that "peace is tranquillity of order," which tranquillity consists in all the appetitive movements in one man being set at rest together.
Reply to Objection 2. If one man consent to the same thing together with another man, his consent is nevertheless not perfectly united to himself, unless at the same time all his appetitive movements be in agreement.
Reply to Objection 3. A twofold dissension is opposed to peace, namely dissension between a man and himself, and dissension between one man and another. The latter alone is opposed to concord.
Source: Summa Theologica (New Advent)