The key strategic point in the Leighrid sector is the world of Berrima Prime. A staging point for trans sector commerce and the main arms manufacturer for the sectors forces. It's fall to a massed raiding fleet struck a devistating blow to imperial operations in the area. The imperial response, led by the Blood Angels Commander Dante, arrived to find Chaos and Dark Eldar forces fighting over the spoils. Seizing the moment, he led two full companies of Blood Angels to spearhead the reclaimation campaign.
Sounds interesting, as everyone knows that the Adeptus Astartes would attack at some randomly selected point with overwhelming force, alerting the Chaos and Dark Eldar, drawing their reinforcements to the region. At which point the Astartes Commander would dust himself off, declare his work done and remove his 7' tall powerarmoured rainbow warriors back to their galactic love nest. So the IG could 'mop up' more of the enemy than were there when the Astartes arrived in the first place.