Battle of Simeis 147

At the height of the conflict between the Forerunners and the Flood, the latter controlled billions of assets that could be fielded against the Forerunner forces - infection became a simple matter of amassing a fleet large enough overwhelm the orbital fleet, at least for a short while, and land enough ground forces to begin the infection process on the planet. As time passed, the Forerunners realised that continuing to battle such a virulent enemy on the ground was completely pointless, and resorted to increasingly extreme measures, ranging from the reduction of a planets crust to a molten state via spaceborn directed energy weapons (a process that would become known as glassing during the Human-Covenant War) to the initiation of a supernova.
As the same time, the Flood had learnt much from its dealings with a civilisation of such sophisticated technology, enough to know that simple head-on confrontations were not enough to gain victory. Certainly, it could afford to expend more resources than its foe, and had the advantage of nearly instantaneous response times and the ability to use the Forerunners' own technology against them, but above all its greatest asset was the Forerunners' own weakness - their hesitation. In the early days of the war, the Forerunners had held back so much out of curiosity of their foe, underestimating its potency - by the time they could bring their full might to bear, the Flood had consumed thousands of worlds and billions of sentient organisms. Even later, when the Flood attacked the Forerunners' own colonies, orbital forces would hold back until civilians had been evacuated before opening fire, allowing the Flood to increase their hold on the planets.
The exact events during the Battle of Simies 147 may never be known for sure. Any debris that would have been left from the battle would likely have been destroyed by the supernova, and all that is known is from brief summaries by Forerunner field commanders, recovered from a small number of artefacts. There was a significant colony present in the area, with a substantial population and built up military infrastructure, making it an irresistible target for the Flood its relentless conquest of the galaxy. Elements of the Forerunner Suppression Fleet arrived to try and deal with the infection, but were already too late - the Flood brought captured weapons and ships to bear on the reinforcements, devastating it in the opening hours. The ground battle continued to rage on, but the commander of the fleet, whose name loosely translates to "Cavalry Commander" or "Decurio," deemed the battle lost. Rather than allow the Flood to consolidate the world, the fleet initiated the supernova of the nearby star - only a handful of Forerunner ships survived this event, but the Flood fleet and the infected planet were both destroyed, denying it to the enemy. The surviving ships would later be incorporated into the Ares Defence Fleet, under LV-666 Curator.
Exactly what drew the Flood to the planet is debatable. Some UNSC and Covenant researchers have suggested that it was a military stronghold, perhaps the Forerunner equivalent of Reach during the Human-Covenant War. Others have suggested that it was one of the Core worlds, inhabited for centuries and supporting the majority of the Forerunner population. Still others take a different view. The files on the subject do not broach the subject - whoever they were intended for likely knew the relevance of the planet, and would need no explanation. Though disappointing, this is not surprising.