Objection 1. It seems that endurance is not the chief act of fortitude. For virtue "is about the difficult and the good" (Ethic. ii, 3). Now it is more difficult to attack than to endure. Therefore endurance is not the chief act of fortitude.
Objection 2. Further, to be able to act on another seems to argue greater power than not to be changed by another. Now to attack is to act on another, and to endure is to persevere unchangeably. Since then fortitude denotes perfection of power, it seems that it belongs to fortitude to attack rather than to endure.
Objection 3. Further, one contrary is more distant from the other than its mere negation. Now to endure is merely not to fear, whereas to attack denotes a movement contrary to that of fear, since it implies pursuit. Since then fortitude above all withdraws the mind from fear, it seems that it regards attack rather than endurance.
On the contrary, The Philosopher says (Ethic. iii, 9) that "certain persons are" said to be brave chiefly because they endure affliction.
I answer that, As stated above (Article 3), and according to the Philosopher (Ethic. iii, 9), "fortitude is more concerned to allay fear, than to moderate daring." For it is more difficult to allay fear than to moderate daring, since the danger which is the object of daring and fear, tends by its very nature to check daring, but to increase fear. Now to attack belongs to fortitude in so far as the latter moderates daring, whereas to endure follows the repression of fear. Therefore the principal act of fortitude is endurance, that is to stand immovable in the midst of dangers rather than to attack them.
Reply to Objection 1. Endurance is more difficult than aggression, for three reasons. First, because endurance seemingly implies that one is being attacked by a stronger person, whereas aggression denotes that one is attacking as though one were the stronger party; and it is more difficult to contend with a stronger than with a weaker. Secondly, because he that endures already feels the presence of danger, whereas the aggressor looks upon danger as something to come; and it is more difficult to be unmoved by the present than by the future. Thirdly, because endurance implies length of time, whereas aggression is consistent with sudden movements; and it is more difficult to remain unmoved for a long time, than to be moved suddenly to something arduous. Hence the Philosopher says (Ethic. iii, 8) that "some hurry to meet danger, yet fly when the danger is present; this is not the behavior of a brave man."
Reply to Objection 2. Endurance denotes indeed a passion of the body, but an action of the soul cleaving most resolutely [fortissime] to good, the result being that it does not yield to the threatening passion of the body. Now virtue concerns the soul rather than the body.
Reply to Objection 3. He that endures fears not, though he is confronted with the cause of fear, whereas this cause is not present to the aggressor.
Source: Summa Theologica (New Advent)