I am going to talk about the wars the Space Marines had. The Space Marines are the army to have as Allies if you want to defend your homeland. Space Marines are a genetically modified race of 'Super humans', which were created by the Emperor, to conquer the Galaxy and defend the race of man-kind.
Space Marines know no fear, and during training have the hardest of courses. During their training they have to live up to wounds that would kill a normal human over and over again, and they would live to fight another day. They can't be harmed by diseases and plagues.
In the grim darkness of the 41st Millennium, heretics, traitors, aliens and Daemons assail Mankind from all sides. Of all the soldiers that defend humanity, the greatest are most assuredly the Space Marines - super-human warriors forged through ancient and arcane science, they are the Emperor's greatest gift to the Imperium and the hope of all Mankind.
Towards the end of the Crusade, a Primarch known as Horus - widely regarded as the Emperor's favored son and made Warmaster upon the Emperor's return to Earth - succumbs to the temptations of the Chaos Gods, sparking an event known as the Horus Heresy. 9 of the 18 Space Marine Legions (together with fully half of the Imperial Army and associated forces) turn against the Imperium of Man.
After a series of vicious and bloody encounters, Horus lays siege to the Imperial Palace on Earth and is defeated in single combat by the Emperor, though the Emperor himself is mortally wounded. With the Emperor effectively dead and Horus' rebellion broken, the Primarch of the Ultramarines, Roboute Guilliman, pushes for the adoption of his personal battle doctrine (known in-game as the Codex Astartes) to organise the remaining loyal Space Marine Legions, where the 9 loyalist legions are split into chapters, each consisting of 10 companies of, customarily, 100 Marines.
Click on: Different Legions to see all the different Space Marine Chapters (Companies)
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