Seijo to Sumeragi Ou no Seiyaku Kekkon Hazukashii no de Seijo no Jiman Banashi wa Shinaide kudasai ne...!

She hasn't even experienced first love yet, and now she's marrying the Emperor (her ex-boyfriend) she just met!? A miraculous reversal of fortune begins! Saint Julietta is forced to marry Lukino, the new Emperor of the Izelta Empire, which is on the brink of defeat, after he brings up a 400-year-old vow. While working tirelessly to protect Lukino and the lives of his people after the defeat, Julietta comes to know the kindness of Lukino, who became Emperor knowing he might be executed!? "We can still win this war." Julietta's hidden power will create a miracle from here! A story of a great reversal between a saint and an Emperor!! (Source: Kadokawa, translated)