Within a year of his parents' marriage Louis was born, but his mother, a strong-willed 19-year-old, disliked Monaco and was unhappy with her husband. Shortly thereafter, she left the country permanently, and the princely couple's marriage was annulled in 1880. Louis was raised in Germany by his mother and stepfather, Count -and later Prince- Tassilo Festetics von Tolna, along with his eldest half-sister, Maria-Mathilde (later grandmother of Ira von Fürstenberg), and did not see his father until age 11 when he was obliged to return to Monaco to be trained for his future princely duties.

Louis' father, Prince Albert I, was a dominating personality who had made Monaco a center of cultural activity and whose intellectual achievements were recognized around the world. Unhappy, living with his cold and distant father, as soon as he was old enough, Louis went to France, enrolling in the Saint-Cyr Military Academy. Four years later, after graduating, he asked to be posted with the French Foreign Legion fighting the wars in the African colonies.

While stationed in Algeria, he met Marie Juliette Louvet (1867–1930), a cabaret singer. Juliette was already the mother of two children, Georges and Marguerite, by her former husband, French "girlie" photographer Achille Delmaet. Reportedly, Prince Louis fell deeply in love but, because of what in those days was seen as her ignominious station in life, his father would not permit the marriage. It has been asserted that Louis ignored his father and married Juliette in 1897: there is, however, no evidence for this allegation. Their illegitimate daughter, Charlotte Louise Juliette, was born on 30 September 1898 in Constantine, Algeria. There is no mention of Marie Juliette Louvet in the authorized biography of her grandson, Prince Rainier III, who is Monegasque by nationality but genealogically is French, Mexican, Italian, German, Scottish and English.

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