I'll repost a post from elsewhere:
It did occur to me that while it looks impressive the Covenant forces that came with arent actually that impressive. On the ground well the covenant are pretty screwed, Even in halo canon they were losing way more troops than they could sustain, they relied heavily on reinforcements turning up to maintain cohesion, the UNSC were usually able to kick their asses in most ground side arenas, i imagine if the fleet fled or was destroy they wouldnt last to long on their own.
As for the fleet, it depends on what both sides have left and their status, The most dangerous unit is obviously the Varic class but again it depends on which version it is, If its the invasion version is significantly less dangerous if its the scout version its middling but if its the flagship version thats a nightmare to happen.
The battlecruiser again depends on what the decided to fight, if the isot happened between Operation uppercut and the covenant reinforcements arriving i would imagine the remain covenant would try and fight but ultimatly retreat quickly. The corvettes were able to be taken on 2 - 1 by UNSC frigates or 1-2shot by cruisers, its why they ended up mostly being used as quick ground invasion support or stealth missions.
At this point while yes some crippled ships remained most of the Defense fleets and late arrivals were still capable of fighting, the flagship if i remeber was still lashed to an orbital but the UNSC also still had most of their defense grids and main station, It was only when the reinfrocements arrive they suffered mass casualties and lost their heavy units.
Ultimatly though the worst thing going for the covenant is the fact they are literally stuck without support or infrastructure to maintain their units, Its a army without support and with a point of view that even some of the worst in Trek wouldnt share and even those who would like the dominion wouldnt stand for a rival and try to kill them as soon as possible. Add in also the usually proof they would use to prove their religion doesnt exaclty exist in the Trekverse and most races who know about the high beings or high tech races would scoff at them,
Afterall the Klingons once had gods(advanced race) and promptly killed them all or chased them off their homeworld. Same with the iconicans and such.
As for the UNSC while yes they are likely to be looked on with suspician and some hesitation, they would still be welcomed into the bosom of most of the alpha races, Trek was very friendly for the most part, Add into the fact the horrors the covenant have being doing it wouldnt take much to get the races onside. Say what you want of the Xenophobic UNSC they still were willing to accept the Covenant races once the war was over.
Also im guessing but it sounds like everything around reach came with, thats going to be intresting as if they survive the immediate event, they are going to be along of UNSC wrecks and personnel they can recover and repair(they never got to in OTL even if some ships were only crippled) infact the flagship survived till nearly the end with crew surviving till it was destroyed along with the rest when the covenant left after glassing the planet.
Add to this Covenant wrecks more importantly the Supercarrier halfs, again lots of tech there to salvage and research which again wasnt possible in Canon.
Now saying that the UNSC was avle to build the Infinity hidden in the ort cloud while fighting the war and equip it with reverse engineered systems from the foruneers. Now if they can do that in war time with scraps they found, imagine what they would do with all that salvage coveant tech and whats buried on reach itself.
i just hope if the corssover gets written the halos and such dont show up for reasons, as it would be odd as hell for that to be a thing.
How many Spartans would be running around during this First Contact between United Earth and Reach? You know, for Augument Trauma.
~26 S-II's, and a gaggle of S-III's+Noble Team.
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