The Human Alliance (H.A. for short) is a secret government sponsored organization dedicated to hunting down and controlling the outbreak of Devil Beasts. Many of their member are scientists or enforcers that primarily compose of Devilmen.
When the Devil Beast Syndrome was discovered many branches around the world came together to put up defenses against the outbreak. The authority has spread to the defense forces of each country. Their task is to contain and control the Devil Beasts in order to keep them from going wild and dominating humanity. The end of the series however reveals that this is really just a facade for its founder, the archangel Michael to put the Devil Beasts close together in order to later eliminate them and any chance of their twin Satan from overthrowing their creator.