Complex sentence: From September 1939, he experienced the concentration camps for stateless persons first at Orléans and Cepoy (near Montargis), then at the Camp des Milles and then Nîmes, from where he escaped with some friends in the course of summer 1940.
Split into simpler sentences: From September 1939, he experienced a series of French internment/concentration camps for stateless persons, first at Orléans and Cepoy (near Montargis), then at the Camp des Milles, and then Nîmes. He escaped from the Nimes camp with some friends in the course of summer 1940.