Complex sentence: He fought in the French army in 1916 to 1920 and again in 1940; and when joining the free French secret service, he volunteered immediately to go back to France as a secret agent.
Split into simpler sentences: He fought in the French army from 1916 to 1920 and again in 1940. Later when joining the free French secret service, he volunteered immediately to go back to France as a secret agent.