Complex sentence: Couturat, who was the leading proponent of Ido, was killed in an automobile accident in 1914, which, along with World War I, dealt a serious blow to the Ido movement.
Split into simpler sentences: Couturat, who was the leading proponent of Ido, was killed in an automobile accident in 1914. This, along with World War I, dealt a serious blow to the Ido movement.