Tenkaichi!!

It's her high school debut, yet Tora Takei can't seem to fit in. Then somehow, she ends up time-slipping into the Warring States period! In order to return to the present, she has no choice but to change history. Disguising herself as a man, Tora gets caught up with Mori Ranmaru, the handsomely beautiful, yet deadly guard of Oda Nobunaga. Slowly, she discovers the kind and honest nature of this strongest and most beautiful of warriors. Falling in love with Ranmaru, the story of Tora's time-slip into the Warring States period begins!

(Source: Jmanga)