On fortitude. This is divided into two parts: as it concerns matters of war and matters at home. The first cannot be a virtue unless combined with justice and prudence. The other depends to a large extent upon endurance.
175. We have discussed fully enough the nature and force of what is virtuous from the standpoint of justice. Now let us discuss fortitude, which (being a loftier virtue than the rest) is divided into two parts, as it concerns matters of war and matters at home. But the thought of warlike matters seems to be foreign to the duty of our office, for we have our thoughts fixed more on the duty of the soul than on that of the body; nor is it our business to look to arms, but rather to the affairs of peace. Our fathers, however, as Joshua, the son of Nun, Jerubbaal, Samson, and David, gained great glory also in war.
176. Fortitude, therefore, is a loftier virtue than the rest, but it is also one that never stands alone. For it never depends on itself alone. Moreover, fortitude without justice is the source of wickedness. For the stronger it is, the more ready is it to crush the weaker, while in matters of war one ought to see whether the war is just or unjust.
177. David never waged war unless he was driven to it. Thus prudence was combined in him with fortitude in the battle. For even when about to fight single-handed against Goliath, the enormous giant, he rejected the armour with which he was laden. His strength depended more on his own arm than on the weapons of others. Then, at a distance, to get a stronger throw, with one cast of a stone, he slew his enemy. After that he never entered on a war without seeking counsel of the Lord. Thus he was victorious in all wars, and even to his last years was ready to fight. And when war arose with the Philistines, he joined battle with their fierce troops, being desirous of winning renown, while careless of his own safety.
178. But this is not the only kind of fortitude which is worthy of note. We consider their fortitude glorious, who, with greatness of mind, “through faith stopped the mouth of lions, quenched the violence of fire, escaped the edge of the sword, out of weakness were made strong.” They did not gain a victory in common with many, surrounded with comrades, and aided by the legions, but won their triumph alone over their treacherous foes by the mere courage of their own souls. How unconquerable was Daniel, who feared not the lions raging round about him. The beasts roared, while he was eating.
Source: On the Duties of the Clergy (New Advent)