Complex sentence: A 1959 agreement between Paris and Buenos Aires instaured a "permanent French military mission", formed of militaries who had fought in the Algerian War (1954-62), and which was located in the offices of the chief of staff of the Argentine Army.
Split into simpler sentences: A 1959 agreement between Paris and Buenos Aires created a "permanent French military mission", formed of French army personnel who had fought in the Algerian War (1954-62). The mission was located in the offices of the chief of staff of the Argentine Army.