Complex sentence: Jacques Foccart remained in service under Georges Pompidou's presidency (1969-1974), and was then replaced by president Valéry Giscard d'Estaing (1974-81) by René Journiac, whom he had formed himself.
Split into simpler sentences: Jacques Foccart remained in service under Georges Pompidou's presidency (1969-1974). Foccart was then replaced by president Valéry Giscard d'Estaing (1974-81) by René Journiac, whom he had formed himself.