The Vanguard Force of four hundred warships swept all before it, conquering one world after another. No world could muster enough strength to resist.
The local governors fled, taking almost all their Planetary Defense Forces and planetary wealth with them. Publicly, they called it scorched earth, denying the enemy any resources. In reality, the gold and silver they carried were useless, while the heavy raw minerals they left behind became vital supplies for the enemy.
Organized by Ecclesiarchy leaders who swore to die rather than retreat, the loyal people of those worlds armed themselves with decrepit weapons found in warehouses, neglected for years, and were then slaughtered by the Iron Warriors.
As for the governors who escaped with vast wealth and armies, aside from those who defected to the enemy, some simply became pirates, while others became the enemy's "Vanguard Force."
They reached the next world first and looted enormous amounts of wealth. That world's governor was also packing up to flee, and the various forces immediately hit it off. The suffering local population lost everything. The only good news, perhaps, was that they stripped the place so thoroughly that the Iron Warriors gained nothing beyond people and heavy minerals.
This ever-growing army fled deeper into the Imperium. Bold souls kept slipping away amid the chaos to become pirates, while the rest continued running under the pretext of concentrating their surviving forces at major military strongholds. Then... they skirted several of those strongholds and kept running.
They ran all the way to our homeworld.
Military movements on such a scale could not escape our forward reconnaissance posts, which immediately sounded the alarm.
These fellows had enough sense to plan on avoiding our territory and heading straight into the next sector. But we did not know that and mistook them for