Yakyuu de Sensou suru Isekai de Chou Koukou-kyuu Ace ga Jakushou Kokka wo Sukuu you desu.

Tokonatsu Taichi took the summer Koshien by the storm as a sensational ace pitcher, with a full arsenal of pitches including a 98 mph fastball. But on the way home from the game, the team bus suffered a tragic accident and Tokonatsu Taichi lost his life... Only to be transported to a new world where all international conflicts are resolved by a game of baseball. Unfortunately, the country he landed in was the weakest, and had seen gradual loss of its resources and people to neighboring countries. Can Taichi turn their fortunes around?
In Another World Where Baseball Is War, a High School Ace Player Will Save a Weak Nation
超高校级投手在用棒球代替战争的异世界拯救弱小国家
野球で戦争する異世界で超高校級エースが弱小国家を救うようです