The enemies of the UNSC are numerous, though each is unique in their objectives. Whatever the motivation, they all became the target of the UNSC and were the focus of many operations to neutralize them.
The most well known enemy factions to humanity is the Covenant. Throughout much of the 2500s, the Covenant spawned some of the most ruthless and murderous villains that humanity has ever seen. Through the entirety of the Human-Covenant War, more than a billion human lives were lost due to the Covenant and their leaders.
Like the Forerunners before them, humanity relies heavily on the effort of artificial intelligences for the bulk of their day-to-day work. And also like the Forerunners, this reliance became a crutch that could be pulled out from under them at a moments notice. While the work of AIs is largely responsible for the rapid rise of both civilizations, the AIs are also the cause of that civilization’s most trying times.
Children of what remained of the Precursors following their destruction at the hands of the Forerunners, the Flood is a curse upon the galaxy that can seemingly never be completely destroyed. Despite their technological prowess, the Forerunner civilization was wiped out by the Flood, and humanity has struggled mightily to contain even the most minor of Flood outbreaks.
Although the Forerunners were millions of years more advanced than humanity, they were still not without conflict. In fact, their conflicts were often of far greater significance than anything humanity has ever seen. Whether it was a conflict with humanity itself like what was seen with the Ur-Didact, or an internal conflict between Forerunners as was the Didact and the Master Builder, the Forerunners created and continue to create much strife within the Milky Way Galaxy.