SEIJO TO SUMERAGI OU NO SEIYAKU KEKKON HAZUKASHII NO DE SEIJO NO JIMAN BANASHI WA SHINAIDE KUDASAI NE...!

Julietta, the saint of knowledge, is forced to marry into the Iserta Empire, which is on the brink of defeat, despite knowing she is being removed as a nuisance, due to a 400-year-old promise. Her fiancé, Lukino, is also a young man of commoner origins who has been elevated to the position of emperor. In the worst possible circumstances, the two are touched by each other's kindness, and eventually perform a "miracle" that turns the tide of the war on its head. "I look forward to working with you, partner." (Source: Kadokawa, translated) — Description from AniList community (anilist.co).