Why does Baron Douphol challenge Alfredo to a duel?

In Giuseppe Verdi’s opera La traviata, Baron Douphol challenges Alfredo Germont to a duel because of Alfredo’s public humiliation of Violetta Valéry at Flora’s ball.1

After Alfredo discovers that Violetta has left him, he becomes enraged and suspects she has returned to her former lover, the Baron. At the ball, Alfredo wins repeatedly against the Baron at the gaming tables and taunts him, suggesting that if he were to kill the Baron in a duel, Violetta would be left without a protector. Following these provocations, Alfredo publicly insults Violetta by throwing his gambling winnings at her feet, claiming he is repaying her for the money she spent on him during their time together. The Baron, acting to defend Violetta's honor and respond to Alfredo's aggressive behavior, challenges him to a duel.1