Battle of Diamat
The Battle of Diamat was a minor engagement of the Horus Heresy.
When word of the Battle of Isstvan III reached the Dark Angels, the bulk of the Legion was bogged down far from Terra in combat against the Gordian League, something Horus had deliberately engineered. As a result, Lion El'Jonson could only lead a small force of sixteen warships against the Warmaster, deciding to make his first move by going to the Forge World of Diamat and denying the heavy siege engines there to Traitor forces.
When the Dark Angels fleet arrived near Diamat, they were confronted by a larger but second-rate fleet of traitor Imperial Army warships led by several Grand Cruisers. Horus had already dispatched his own forces to Diamat to secure the war engines. After a tense naval battle, the outnumbered Dark Angels prevailed with minimal losses thanks to the tactical abilities of their Primarch. The Dark Angels then landed at Diamat's principle forge-city, sweeping aside what little resistance they encountered. After this the traitors organized a major counterattack, this time led by Sons of Horus Marines. This, compounded with the betrayal of the planet's ruling Tech-Priest Archoi and his Skitarii, made defeat seem imminent for the loyalist forces. The Lion only turned the tide by destroying the primary forges of the planet as they were being overrun, annihilating the traitors and denying Horus their manufacturing ability.
In the aftermath of the battle, the Iron Warriors led by Perturabo arrived over Diamat on their way to Isstvan V to engage in the Drop Site Massacre. Having secretly pledged himself to Horus, Perturabo lied and maintained a facade of loyalty to the Lion. Seeing the siege engines as more useful in Iron Warriors hands, Jonson unwittingly gave them over to the traitorous forces.