Until 1993, I believe it was, any child born in France automatically acquired French nationality. The number of immigrants and their descendants from the Muslim Maghreb region had increased dramatically in France, and not all these French citizens were actually loyal to France. At one Algeria-France game, the French-citizen, Maghreb-descended crowd cheered against the French team. It was that kind of incident, which made many franco-français angry, that provoked the withdrawal of automatic citizenship for children born in France to foreign parents. Although the law was aimed at the Muslim population, it applies, of course, to everyone. Children of foreign parents must apply for French citizenship when they turn 18, and it can be denied if they are not considered French enough. I know a Paris-born-and-French-school-raised American girl who was turned down because she went to an American college.
A was born in Paris but after this law was passed, and hasn't got French nationality yet. (By the way there is no contradiction to American law; dual nationality is perfectly legal in the U.S.) She flew home to France recently from the U.S.A. and wrote, "The man at passport control saw that I was born in France, and spoke to me in french for a bit, and then said 'Bienvenue chez vous, mademoiselle A*' despite my american passport which i thought was so lovely."
Notice how A spells "american" and "french" without capital letters, the French way.
*Welcome home, Miss A"