Complex sentence: His life was once again affected by war in 1808, when he left Vienna when it was occupied by the French under Napoleon and returned to Paris, where he spent the rest of his life teaching composition and in 1818 was appointed professor at the Conservatoire.
Split into simpler sentences: His life was once again affected by war in 1808, when he had to leave Vienna. Reicha settled in Paris, where he spent the rest of his life teaching composition; in 1818 he was appointed professor at the Conservatoire.