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The military forces of the indigenous Istvaanians were professional soldiers who did not break easily, even when confronted with the superhuman might of Space Marines.
There was some resistance, but the last elements of the aggressive faction opposed to Imperial Compliance were destroyed by the Raven Guard at the Redarth Valley on Istvaan III.
Praal was actually leading the revolt, and the streets of Istvaan III's planetary capital, Khry Vanak (the "Choral City," in Imperial translation) were running red with the blood of non-believers.
Though they had waged war against one another, the Imperial Truth had prevailed and the Istvaanians had sworn to accept its teachings.
By proving themselves men of wisdom and civilisation, they were deemed fitting partners for the many other worlds of the nascent Imperium.
The destruction of Istvaan III by virus-bombing and the subsequent internecine battle that followed between the Sons of Horus, Death Guard, Emperor's Children and World Eaters Traitor Legions and their own Loyalist elements was the first overt sign of Horus' rebellion against the Emperor and turn to Chaos, and the first battle of the Horus Heresy in the early 31st Millennium.
An ancient Terran proverb once said that all wars, no matter how bloody or great, begin with but a single shot fired.