Biggles and Ginger join the French Foreign Legion, and assist Marcel Brissac in bringing down an organisation dedicated to fostering conflict and war. Von Stalhein turns up as one of the enemy to continue his battle with Biggles.

goodreads summary

When Biggles' opposite number and old friend in the French Surete, Marcel Brissac, joins the French Foreign Legion, Biggles is intrigued as to why. After thinking it through, Biggles concludes that Marcel is onto an international gang with a financial interest in war. Various Countries are at war with each other and when there is a chance of peace, an incident usually occurs to thwart the peace process. The international gang is behind such incidents. The only lead is the knowledge that the gang encourages French Foreign Legionnaires to desert and join them.