Why is Roxanne so concerned when she learns that de Guiche is about to order the soldiers to depart for war? What trick does she play as a result of her concern?

De Guiche tells Roxane he is heading off to war and wanted to say goodbye to her. She becomes horrified when she hears the Guards, including Cyrano and Christian, are going too. De Guiche takes this for concern about him and is pleased. Roxane tells De Guiche to take revenge on Cyrano by not letting the Guards go. De Guiche is tickled by this idea, not guessing her reasoning. He waxes poetic about his love for her and she plays along. He departs.