When Equestria formed, the unicorns and earth ponies saw stallions being conscripted into the military as barbaric. Stallions were meant to be protected at all costs, after all! After much debate, Princess Luna offered a solution that appeased all sides. The castle garrison would be a predominantly stallion post. That way, stallions with military-based cutie marks could serve and fulfill their destinies, and the alicorn sisters could protect the stallions in turn.