Game no Sekai ni Tensei shitara Ore ga 〇〇 ni Naru made

After his grandparents—the last of his family—pass away, Kariya finds himself free to discover what awaits outside the harsh confines of his remote mountain village. However, beyond its borders lies a world embroiled in the fires of war, and Kariya is conscripted into the army before he can take his first step toward freedom. It doesn't take him long to realize the truth: this world is a near carbon copy of a game he played in his previous life. When his identity as a reincarnated player comes to light, Kariya is tasked with a perilous mission to escort Wald, a handsome prince disguised as his twin sister the princess, safely through enemy territory and back to the royal capital. But Kariya isn't the only one who knows the secret of this world—and having the combat proficiency of a common NPC won't keep anyone safe. Armed only with player foresight and the skills of an S-tier craftsman, he'll need every ounce of cunning he can muster to turn the tides of battle and bring his prince home.

(Source: J-Novel Club)