Russell Shaw on prudence:

People who want to understand prudence will do well to study St. Thomas More, a man who didn’t wish to be a martyr but became one anyway because he was convinced he had no other acceptable choice.

Prudence is widely misunderstood today. In everyday speech it’s a synonym for caution, even timidity, and by no means a character trait to be admired. Josef Pieper, a German Catholic philosopher in the tradition of St. Thomas Aquinas who has written wisely and well on the subject, remarks that in many people’s minds “prudence seems less a prerequisite to goodness than an evasion of it.”

But that misrepresents prudence in its classical sense. St. Thomas Aquinas calls it “absolutely the principal of all the virtues.”

What is prudent is neither necessarily nor even often, maybe, what is expedient.