Spartans are the best of the best when it comes to dealing with the Covenant, the Flood, or anything else that threatens the safety of humanity. It's the reason why Master Chief has always been the frontrunner for the Halo series, and for good reason. Apart from ODSTs, Spartans are the tip-top of active duty personnel in the UNSC. At least until the events of Halo: Reach, most of humanity would've assumed all Spartans were unstoppable. Interestingly enough, the Spartan program actually wasn't designed to deal with extraterrestrial threats like the Covenant or otherwise, though they ended up being very good at doing so.
In fact, Spartans were primarily trained and augmented to participate in the Insurrectionist War, a civil war within humanity's many intergalactic colonies. As part of the lesser known lore of the Halo universe, the Insurrectionist War was a significant armed conflict that started three decades before the Human-Covenant War. While a precursor version of Spartans had existed prior, the same Spartan program where Master Chief came from was initiated because of the Insurrectionist War. It's not an often talked about portion of Halo's history, especially in the games, but it's a very intriguing and unexplored portion of Halo lore that'd be interesting to spotlight.
Halo's Progenitor Conflict: What is the Insurrectionist War?
The Insurrectionist War officially began in the year 2494, about 30 years prior to the beginning of the Human-Covenant War in 2525. Portions of human colonies had sought independence and even rebelled against the UNSC and United Earth Government prior to 2494, but the rebellion was considered an organized effort that year. While Halo games haven't particularly referenced the Insurrection outside of offhand comments, it's a very important conflict for both humanity's history, and more specifically the history of the Spartan program. In a way, Insurrectionists may have indirectly spurred on humanity's best chance at beating the Covenant 30 years later.
Rising up in the Outer Colonies, Insurrectionists had decided they didn't wish to participate in the Unified Earth Government or abide by UNSC regulations. Many colonies demanded independence from a government that many believed was totalitarian, while the UEG categorized insurrectionists as rebellious separatists. By the time 2494 came around, groups like the United Rebel Front and Freedom and Liberation Party had formed and began a slow and arduous uprising against the UNSC. The military arms of the UNSC would retaliate by initiating Operation: TREBUCHET, an interplanetary mission to put down the Insurrectionist rebellion in the Eridanus system.
Halo's Master Chief Was Designed to Fight in the Insurrectionist War
Partly before Operation: TREBUCHET, the UNSC's Office of Naval Intelligence had secretly convened to bring back members of the ORION Project (under the leadership of Dr. Catherine Halsey) to initiate another new project. The SPARTAN-II program was to build upon the previous ORION efforts, though in a much larger way. ONI special forces kidnapped and indoctrinated 75 young children into the new Spartan program, for the explicit objective of crushing the Insurrectionist rebellion with force. And that was largely what they did until 2525, when humanity made first contact with the Covenant at Harvest and began the Human-Covenant War.
From there, most Halo fans know the rest: Many years would go by, the Covenant would declare a holy war and enact a genocide on humanity, with the events of Halo: Reach and subsequently Halo: Combat Evolved kicking off the main story. Because of the severity of the Human-Covenant War and the perspective of the Master Chief, players just didn't learn much about the Insurrection unless they looked for it. However, the Insurrection is the inciting conflict that created the SPARTAN-II program, of which humanity would've been wiped out by the Covenant without. Like Halo 3: ODST, it'd be interesting to see a spin-off that takes place during the Insurrection.
There's a whole 30-year gap of time where a lot of big developments for humanity take place. The very strength of humanity's military force was designed to specifically tackle the Insurrectionist conflict, even though many would've assumed Spartans were trained on Reach to deal with the Covenant. SPARTAN-IIs were meant to crush a human rebellion, which is kind of ironic given the context of the games. It's hard to say what kind of spin-off that would even look like, but it'd probably be closer to a Halo 3: ODST experience than anything else. There's plenty of intriguing history in those 30+ years of Halo lore where humanity was far more divided than ever.