Nise no chigiri manga

Nise no Chigiri summary:

Mana is an ordinary high-school student who somehow travels through time to the Sengoku Jidai (the Warring States Period) and encounters Uesugi Masatora, one of the generals. He decides to protect her, while his rival Takeda Shingen targets her. With Mana knowing the outcome of this war, will she change the future?