Briefly about Aku no Joou no Kiseki manga:
By the time she realized her position, a college student, Mary Toujo, had fallen on the battlefield. Looking back distractedly, she saw a large crowd of knights in Western-style armor. They called Mary “Her Majesty”. She seemed to be in the middle of a war, and her forces were at a disadvantage.
Read Aku no Joou no Kiseki Manga.
Sure it was a dream, she made a plan and won, but… somehow she really replaced the Queen!? After that, she realized that the cause of the war was the queen’s bad reign and that she couldn’t go back to her original body. So Mary has to rebuild the ruined country?