Then the capital of Britannia is vaporized. The floating fortress, Damocles, is loaded with annihilation bombs.
Schneizel and Cornelia never returned to the homeland, but they give Lelouch a call to contest his authority. They have an alternative candidate.
It's Nunnaly.
The Britannian homeland is in disarray. Lelouch cannot requisition more troops on a reasonable timeframe.
That means this is the decisive battle. Lelouch and his current force vs the Black Knights and Schneizel's Damocles.
Winner take all. The entire world.