Early in the development of his theory Freud saw sexual energy as the only source of energy for the id. After the tragedy of World War I, however, Freud felt it necessary to add another instinct to the id. So, he proposed thanatos, the death instinct. Thanatos accounts for the instinctual violent urges of humankind. Obviously, the rest of the personality would have somehow to deal with these two instincts. By adding thanatos, he could describe more mental phenomena. His idea of id had wide influence.