The Mazin War is a major event in Shin Mazinger ZERO. It was the battle of the desperate forces of the remaining militaries and only machine capable of fighting against the Mechanical Beasts, Mazinger Z.
The maniacal genius, Dr. Hell had uncovered the ancient weapons of Bardos the Mechanical Beasts and used a weapon to control them, the Bardos Wand. With the Mechanical Beasts and the Bardos Wand's ability to control nearly all forms of technology, Dr. Hell had effortlessly conquered most of Earth and over powered every military weapon the world threw at his Underground Empire. The only exception was in Japan where the robot Mazinger Z created by Dr. Hell's former colleague Juzo Kabuto at the cost of his life was piloted by Juzo's grandson Koji Kabuto to fight against Dr. Hell and the atrocities he committed.
The Mazinger fought many battles against the Underground Empire's forces, at times being joined by Aphrodite A and Boss Borot. Eventually however, Dr. Hell decided using a full on assault to attack Japan with an entire army of Mechanical Beasts and the military weapons he took from the world's militaries. The opposing side got help from the remaining armies that survived the onslaught of Mechanical Beasts as well as the newly built Mazinger Army. Both sides met in battle with the Mazinger Army completely eliminating all Mechanical Beasts but at the cost of their commanding officers save Sayaka who managed to escape the destruction of Venus A. Koji confronted Dr. Hell for a final showdown but was completely overwhelmed and became mortally wounded, eventually dying from his wounds. However this gave the Photon Power Laboratory just enough time to bring up a giant Photon Barrier around Japan's Kanto Region to block out any further Mechanical Beasts.
The Mazin War was a major turning point in human history with the surviving people viewing Mazinger Z as their savior. Koji was brought back to life as a cyborg with help from allies who would fight to the best of their abilities for Japan. The barrier however did not completely stop the Mechanical Beast as one to three of them always manage to sneak past the barrier but never an army. Time later proved to be both advantageous and dangerous for Japan as Minerva X had joined the Photon Power Laboratory but the Underground Empire managed to break through the barrier through a suicide run.