"I think you don't realise something Cadence," Luna said after a minute. "Choosing to make Canterlot our last stand isn't something I chose lightly. Put simply, we do not have the strength to fight the Mareitanians on even terms. They could wipe out our forces, conquer Equestria, and destroy so much doing so, all in a matter of weeks. By that time we wouldn't have the strength to defend Canterlot, whereas right now we could hold the city for quite some time. Or so I hope. Time enough for Twilight to succeed, provided she is not distracted by further catastrophes."