Metsubou Kokka no Yarinaoshi: Kyou kara Hajimeru Gunshi Seikatsu

Ludek, a nation that possessed immense wealth and unparalleled military power, was destroyed. Roa, a young man who was an ordinary civil servant in that nation, miraculously survived, but he grew older burdened by guilt for abandoning his friends and surviving alone. But one day, he wakes up to find himself two years before his homeland's destruction! To prevent the downfall of his country and protect his comrades, the civil servant becomes a strategist, using his memories and the experience he gained in the future as his weapons, and confronts the fate of destruction! A rags-to-riches war tale of a former ordinary civil servant!!

(Source: Square Enix, translated)