system
Given the following post, respond as if you're commenting on Reddit:
human
[Halo]Are UNSC Marines and personnel (not including Spartans) really that incapable of dealing with Alien military threats?

I'm going off the games. I haven't read the books, so I'll gladly welcome that information.

The Covenant and the Flood have some frightening advantages - be they biological, physical, or technological. I can see how humanity is fighting a losing battle. However, at their best, I've seen marines barely manage to survive a coordinated attack with the help of John-117. Additionally, they don't seem to have any actual victories over the Covenant outside of the Halo game canon - certainly none without Spartan help.

The dichotomy between their ineffectiveness as military units and the Spartan's overwhelming power is also strange to me. I know that Spartans are supersoldiers with powerful armor, yet most marines can barely go toe - toe with an Elite while one Spartan almost single-handedly won the human-covenant war. 

Side note: Maybe marines are more competent in the books?
gpt
The thing to remember here is how much more advance the Covenant are than the UNSC.  Marines aren't Elites, they're Grunts, or Jackals at best.  Elites are equivalent to Spartans.  The one Spartan you speak of was a very good Spartan, more equivalent to a Covenant Ultra, and lucky enough to be in the right places at the right times.