Runin

From MangaHelpers:
Humankind ignored Earth's screams of pain. The result: devastating earthquakes and unknown viruses brought the human population to their knees. As money became as worthless as a piece of paper, people turned to war and violence as a means to survive. Nuclear war was the inevitable conclusion to the loss of civilization.
Our story begins in the ruins of Manhattan, now only a decaying mass of the rotting dead and the rampant wilderness. Thieves and murderers run free as if they rule the land under the banner of "survival of the fittest." One of the last living survivors of the Japanese race, Takeru, trains under his master in the ways of the Japanese samurai to bring justice and peace to his friends and family.