So its a time travel manga with a male mc, where the MC is in high school and play Kendo, and he suddenly transported to history of japan in the middle of war, he eventually became a good swordman and suddenly goes back to his world, still a high school boy even though in other world he has spents years and years.

I don't know if these are what you are looking for, but here my findings thanks to ChatGPT:

https://myanimelist.net/manga/132556/Shin_Gunjou_Senki

https://myanimelist.net/manga/7325/Sengoku_Strays?q=%E6%88%A6%E5%9C%8B%E3%82%B9%E3%83%88%E3%83%AC%E3%82%A4%E3%82%BA&cat=manga