Shin Gunjou Senki

A battle between modern-day thugs and Sengoku warlords!!

"What do you think is the difference between a murderer and a wartime hero?" This is the question put forward to Michiro, a college student and aspiring mountain rescuer, by his younger brother Makoto, who is being held in prison on suspicion of the attempted murder of a friend. Michiro replies that it is a matter of justice, but Makoto believes that it is simply a matter of the era. As Makoto is handed an unsuspended sentence and Michiro is at his wit's end, suddenly, a red rain begins to fall, and the area is enveloped in fog. Thus, Michiro, along with the entire prison, is sent to the Sengoku Era...

(Source: Shueisha, translated)