Royal Recruit by Susan Grant

When an upstart little planet called Earth threatens Queen Keira with its fleet of spaceships, the fate of her people lies in her hands. She must bind herself to a barbarian prince from the rogue world—or face the destruction of her own.
Fighter pilot Jared Jasper knows the trick Earth pulls on the alien invaders isn’t a permanent solution. What they need is a peace treaty. It just never occurs to him that he will wind up on the bargaining table.

But before he can say “I don’t”, he’s wedded to the galaxy’s goddess-queen. With billions of people depending on him, he doesn’t have a choice—sleep with the enemy . . . or bring on interstellar war.
An arranged marriage with a twist, this galactic alliance may just turn into a love match.

(Previously published as My Favorite Earthling and briefly as Royal Recruit)