To the extent of splitting the Atom.
We've seen several times how much destructive energy a massive build up of Magic can achieve. We've seen a natural Crystal, the Aeon Stone, harness and amplify Magic to a point of critical mass where it detonates with catastrophic force. A confined Aeon explosion at the end of the Season of the Dragon obliterated an island. It was hinted an unrestrained explosion would decimate a portion of the Continent.
Tamriel, in essence, has Weapons of Mass Destruction, and with Magic being an abundant resource in this world, it's safe to assume if the powers that be had the mindset to order their Mages to do so, creating these weapons would be easy. An appropriate chunk of Aeon Stone given a magical catalyst has the capability to destroy armies, and Elsweyr is, by our knowledge, the only place on Tamriel it's found.
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