In the article: S Mandelbrojt, Souvenirs à bâtons rompus de Szolem Mandelbrojt recueillis en 1970 et préparés par Benoit Mandelbrot, Cahiers du Séminaire d'histoire des Mathématiques 6 (1985), 1-46 Szolem Mandelbrojt, in a conversation with his nephew Benoit Mandelbrot, explains why and how a Pole became a French citizen:-

I want to tell you how I got myself naturalized in 1926. It was quite an amusing thing. Hadamard liked my work a lot. Quite simply (it doesn't matter if I brag) I was considered a very good mathematician. In 1923 when I did my thesis, Sergi Bernstein was in Paris. Everybody knew that I knew him. Hadamard and Montel, being rather naive in these matters, told Bernstein: "Now that Mandelbrojt has done such a beautiful thesis, he will get a professorship at the University of Warsaw". I had never spoken of such a thing. Bernstein, who was Russian, not Polish, told them: "Listen, there is a very deep-seated anti-semitism in Poland, and he will never become a professor, or at least, not quickly. You shouldn't count on it. There isn't one Jewish professor there." After that, Zaremba, who was a good Polish mathematician, quite a lot more liberal than the others, and a Professor at Krakow, came to Paris. Hadamard said to him: - "Really, can't he get a professorship?" "No! If you want, I shall make him assistant". That was in 1924. Hadamard was really offended; Montel also. They asked Mme Curie, because she was a Pole, to intervene. But she explained to them that there was no hope in Poland. So Hadamard told me to become naturalized...

So, a little later, when I was in Rome, I told Hadamard that I would be happy to become naturalized. Hadamard talked to someone at the Ministry of Justice. There was a special law which allowed naturalization of anyone who had rendered important services to the country. It was decided that I had rendered important services to the country. A propos this, I remember something very amusing. One had to go to the Council of State, otherwise you had to wait five or six or ten years.

I went to the Council of State and a councillor of State called me.
"They tell me you are a very good mathematician. Are you like Pascal?"
"No, Monsieur, Pascal made a discovery when he was twelve years old, I wrote a thesis (a good one, it seems), but I was twenty-four".
"Do you know Langeron?"
At that time there was a prefect in Paris called Langeron. "No, I don't know Langeron."
"Every intellectual in France, every intellectual should know Langeron. He is one of the greatest physicists in the world".
"You mean Monsieur Langevin".
"Oh, yes, of course".

I got engaged to Gladys in 1925, but I didn't want to become naturalized by marriage; once you're married it was easy, but I wanted to become naturalized before, because I could do it by the special law. I became naturalized on May 11th, 1926 and I got married on May 25th, 1926, as a French citizen. On the day of my marriage, I received a telegram to go to the Rice Institute. Lovett didn't know my address in Paris, but he knew Hadamard's address, because Hadamard had been there several times. Hadamard was a witness at my marriage and the same day, he brought me the telegram to go to Rice.

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