Operation: ABOMINATION

Operation: ABOMINATION was a failed UNSC expansion into the Perseus Arm of the galaxy. Spearheaded by three ships, the UNSC Apparent Vagueness, the UNSC Ring of Life, and the UNSC Edge of the World, they would use reverse-engineered forerunner slipspace drives to defeat the Covenant splinter faction, the Gods of Orion, who by that point had built their own fleet of forerunner ships. While the space battle had led to a stalemate, the capital world of the Gods of Orion was initially taken by the UNSC successfully, up until the attack on the Covenant's last force, which led into a month-long chase led by Spartan-IV Anwar Helius.
During the battle, Anwar and his forces pursued the Covenant across the planet, finally leading up to a group of covenant making a quick stop at a forerunner temple. During their investigation, the aliens came across a room filled with flood infection forms. Strangely enough, the infection forms seemed to ignore the shielded elites, who recognized the threat and tried to alert the rest of the Covenant forces on the world. But alas, it was too late. The infection forms went for all of the non-shielded covenant aliens and some captured UNSC forces. The elites attempted to rid their ranks of flood, only to get infected as well in the process. As the UNSC forces began to catch up with the Covenant, many began to notice the flood spores in the air, and soon enough many went in wearing gas masks and air filters. However, it was much worse than expected. Already, waves of Covenant had been infected, and both sides entered a large-scale battle nearby the forerunner temple. Worse was the fact that many of UNSC air filters and gas masks were faulty, and there was even a shortage of these masks among the forces. In short, the latter half of the operation was submerged in anarchy. 
A week later, a GO ship was infected after landing on the surface, and as it exited the surface it was quickly shot down by the UNSC. However, the infected debris collided into the Covenant fleet, and in only a few days many of the ships were under the process of infection. After almost a year of the UNSC forces trying to keep the situation under control, reports of a fully-fledged gravemind began to come through, and the UNSC began getting ready to bombard the planet from orbit, and readied to evacuate its surviving ground forces from the surface of the world. Because of this, a set of Pelicans was sent down to collect surviving high-priority targets. Due to the ill-advised actions of several unlucky marines who had forgotten their air filters, they all ended up breathing in flood spores and were slowly infected as they left the surface of the world.
The end of the Operation: ABOMINATION resulted in every ship in the system being infected by the flood. Overall, it was a failure from both sides of the fight, and subsequently led to the system in question being blockaded by the UNSC. However, it spiked quite a lot of intrigue within the UNSC navy of the Perseus Arm, and this fascination spread to many civilians, despite ONI's hiding of the Operation from the public. This fascination led to the beginning of the Perseus Expansion.