La Légion étrangère
It requires only, that France have researched Army Professionalism. It gives 0.01 prestige pr. month and 1.0% immigrant attraction with no real drawbacks. Despite what would be logical, there is no special attraction towards soldier POP's and all foreign POP's are attracted the same.
Since the effect is based per month, it is best to take the decision as early as possible in order to maximize the effect.
The ingame decision text states: La Légion étrangère', known in English as 'The Foreign Legion', is military unit of the French army, with a unique distinction in that it accepts its recruits regardless and their national and personal backgrounds. While originally established as early as 1830, the Legion did not win the lasting legacy it enjoys today until it had seen action during, among others, The Spanish Carlist Wars, The French Intervention in Mexico, the Batter of Camarón, The Franco-Prussian war and, in fact, almost every conflict involving France up until and through the modern era. The image of the young man, perhaps on the run from some personal tragedy, oppression, debt or a criminal past, who joins the French Foreign Legion to serve a tour of personal redemption in the deserts of French Algeria, has since become one if the most iconic and enduring images of the 19th century. Promote the Legions, and see recruits flock to our banners.