When Naponeon invades without warning, and Canterlot's walls crumbling, the Princesses of Day and Night have no choice but to stop one evil with a slightly lesser evil.

Here's an interesting fact: Discord is the Equestrian Spirit of Chaos. He doesn't much take a fancy to foreigners. Especially not those trying to invade.
And Discord really doesn't like the smell of Epoisses au Marc-- Which just happens to be Naponeon's favorite cheese.