The first Invasion fleet was meant to cripple the Helghast defense network, remove the Imperial Fleet as a threat and clear the way for the ground force that was to be sent by the 2nd ISA Invasion Fleet, formed by Battle Group Mandrake among other Battle Groups.
The ISA greatly underestimated the Helghast defensive capabilities, and lacked any knowledge of the Arc Tower defense masterminded by Colonel Radec. As the ISA Cruisers descended into orbit to begin their bombardment of Helghan, the Arc Tower defense grid activated and nearly the entire first wave was wiped out in a matter of minutes by the well-prepared defense of Helghan. However, some of the ground forces managed to deploy as their ships burned, and while they too suffered severe casualties, they managed to (barely) secure several beachheads to the second wave to land upon. Despite heavy losses due to the Arc Cannons, thanks to the mercenary Arran Danner, a portion of the Arc Cannons were destroyed, which opened a window for the rest of the ISA invasion force.