"Sunset." Rose's tone was patient. "Think. The enemy is about to take the capital. Their next step is to secure and consolidate. How long will it be before troops are here in Ponyville? How long will it take them to sack this place? Once they have, how long will it take them to figure out the portal? They probably couldn't mount any kind of serious operations through it, but do you want to see Canterlot High become a literal battleground? What if they realize they could send agents provocateur through it instead?"