Battle of Isstvan III

The Battle of Isstvan III was the first battle of the Horus Heresy, carried out by the traitor legions to purge any remaining loyalists from their ranks. The primarchs ordered all space marines they suspected of being loyalists down to the surface of Isstvan III under the guise of putting down a rebellions, and once they were there they bombarded the planet with virus bombs. While the loyalists were caught off guard and a good number of them were killed a sizable force of them survived the initial attack. Before a second bombardment or exterminatus could be organized to wipe out the the survivors Angron led his legion down to the surface to kill the survivors, forcing the other traitors to hold their fire to avoid killing Angron or his forces. This delay led to a short ground campaign against the survivors, costing the traitors valuable time. While the traitors won the battle, their plan was ultimately a failure. While the loyalists were removed from their ranks and completely annihilated the larger goal of preventing the traitors from warning the Emperor failed, as several escaped to do just that.