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.
Lelouch breaks into the Damocles, properly predicts Schneizel's plan, and geasses him.
But Schneizel doesn't have the detonator for the annihilation bombs. Nunnaly does.
Lelouch has to confront her.