Sei Michaela Gakuen Hyouryuuki Ii

In the time of the crusades, we see a guiltless solder who was betrayed. He jumped into his death but he vowed to everyone that he will return as an angel! Now, he is reborn as a woman who goes by the name of Aiko. He ends up going to war but the memories of the past life are haunting him! The plot is great!