Actually in the ancient world Elephants were considered to be powerful weapons by everyone that had them. They were used extensively in the Mediterranean area until the north african forest elephant was actually driven into extinction by overuse in war. They continued to be used in India & parts further east until the 19th century.
Elephants could be beaten, but it took well-drilled infantry (in large numbers) and often special tactics to handle them. It's fashionable in some books to scorn elephants in combat, pointing out the important battles in which they failed. But these battles are remembered today precisely _because_ of their unusual nature. Ancient users of elephants included both Hannibal and Pyrrhus of Epirus (Pyrrhus smashed a Roman army with them), and these guys weren't dumb when it came to war.
Elephants were risky, though -- when things went wrong, they sometimes turned on their own side. Which is why they're easy to convert I guess.