"When he woke, he saw chaos. Gigantic beasts, mountains of flesh rolled through the camp, barely controlled by their own trainers." (Egyptian Scribe at the battle of Sevoria)
The use of Elephants in war is a fairly recent innovation, dating to the Nile War. There, it was used by the Axumites in an effort to scatter the Egyptian armies. Before Axum's econmy crashed, it worked. These Elephants came from Ethiopea, the traditional region of Axum, where the locals knew how to handle them. Even though controlling them in war was difficult, war elephants where part of the reason that Axum and Kush managed to hold off the Egyptians on the Nile for so long. After the Nile War, the technology for them spread throughout the Southern Nile. It is notable that even though the Indian Elephant can also be domesticated, even better than its African cousin, it has so far not been used in warfare.
They are likely to be used agian in the Great Levantine War.