The plot happens in AU where the sonic rainboom did not happen, but there is no enemy in this, Princess Luna never became Nightmare. So the girls had different lives.
In this line the story Twilight tells that when she failed her test on the day of her exam due to frustration, she breaks the spike egg of another with the sword of her brother, demonstrating a great talent for combat, which makes her take life military and become the vice captain of the army of celestia.
this was a Spikex Twilight story