Academy Cheonjaekaljabi

Following a momentous battle against giants for the fate of the world, Ronan emerges victorious as the last man standing of the imperial army. As he laments this hollow victory with his dying commander, Grand Marshal Adeshan, they quickly learn that the war is far from over and an overwhelming force of giants descends from the heavens. Fearing the worst, yet seeing hope in the courageous Ronan, Adeshan uses her last time slip to send him back to the past, so that he can enroll in the prestigious Phileon Academy and become the strongest swordsman the empire has ever seen!

(Source: WEBTOONS)