Koezaru wa Akai Hana: Taiga ha Mirai wo Tsumugu

In a land where a catastrophic disease has made women vanishingly rare, Naala grew up in the southern country of Ruse watching her mother be taken by soldiers from the northern kingdom of Nasula. The only constant since then has been Olli, her childhood friend and the prince-in-waiting who now calls her his fiancée. When Nasula's army returns for another raid, everything changes in one night.