The Imperial Guard is the largest fighting organization in the galaxy, consisting of hundreds of billions of men and machines drawn from every planet in the Imperium. The basic unit of the Imperial Guard is the regiment, consisting of perhaps as many as ten thousand men raised from a single planet, and operating as an autonomous army, with its own supply chains, support units, and combat forces. Imperial Guardsmen seldom serve on their planet of origin. The only things an Imperial Guard regiment does not supply for itself are air power and space travel; which as a result of the Horus Heresy are relegated to the Imperial Navy, completely separate from the Guard. The Imperial Guard do not feature in Battlefleet Gothic, as they only fight on the surface of planets. The Imperial Navy is permitted to maintain marines and atmospheric aircraft, but other than that its duties are relegated to the vast darkness between the stars.