I Am Not a Tyrant

Yang Kuang (commander of the modern Marine Corps Wolf Fang Brigade and national martial arts champion) is hit by a grenade during an anti-terrorism mission and, on the brink of death, his consciousness travels back to 612 AD (the 8th year of Daye in the Sui Dynasty) into the body of Emperor Yang of Sui, Yang Guang. At this time, Yang Guang has just suffered the disastrous defeat at the Salsu River during his second eastern campaign against Goguryeo, losing over half of his million-strong army, and is on his way back to the capital. As lightning flashes above the imperial carriage, Yang Guang awakens—Yang Kuang's consciousness has replaced Yang Guang's soul, but inherits all of Yang Guang's memories. — Description from novelcodex.org