Actually, it was Tamriel as a whole, perhaps the rest of Nirn too.
You can here NPCs talk about how Ald'ruhn was destroyed by the forces of Oblivion.
And if it were only Cyrodiil, why not send a few Legions from one of the provinces to Cyrodill hmm? Unless, of course, there were ongoing invasions there too. It was already stupid of Ocato not to send a few legions from elsewhere to protect the capital province.