There once existed a bird of massive size, aptly named the Elephant bird.
Over the course of history humanity rode several kinds of animals to war: horses being by a wide margin the most common. However, no large birds were ridden by ancient people.
The still-living example of elephants birds, the ostrich, can run at speeds of up to 70 kph. A horse can travel only 48 kph. Thus, it is safe to assume that an elephant bird would also be faster. What's more, an ostrich's kick has been known to be able to kill a lion - and the ostrich is the smaller, weaker version of the Elephant Bird.
Modern people have proved that ostrich riding is possible. So I was wondering, could Elephant Birds be domesticated and/or trained? If so, would they be feasible as warmounts?