Sousei no Tenkentsukai

In his past life, Chou Sekiei was the undefeated war hero Kou Eihou—no pressure, right? Now, a thousand years later, all he wants is to get a nice, cushy job as a civil official and build a quiet life for himself. Too bad that he still has fighting prowess he can't explain. Too bad that as a child, he was taken in by a renowned general and trained alongside Chou Hakurei, famed for her talent and beauty. Too bad that he's gotten into rescuing pretty (and pretty immature) merchant prodigies. Sekiei was really looking forward to some peace and quiet so he could kick back and read, but when the foreign nation of Gen launches an invasion that triggers war, he's forced to put his books down. With the power of skills he'd hoped never to put to use again, Sekiei will once more have to make a name on the battlefield... Will he and his friends be enough to save their homeland?

(Source: J-Novel Club)