This story is a sequel to Luna's Daughters
(Can be read without first reading the prequel.)
Centuries ago, in the years after Nightmare Moon's banishment, Luna and Celestia's descendants helped Celestia build the new nation.
Luna's great-granddaughter, Gale Glider, forged the Royal Equestrian Coast Guard, imbuing it with traditions of valor, sacrifice, and bravery. Her final words remain their sacred oath:
"We have to go out. We don't have to come back."
Preread by Sledge115 and RDT, thank you!
Cover art found on Google without any attribution, alas.