The alliance would win.

If bringing an ally is allowed they would still win.

The UNSC was able to fight a 25 year long conflict against an enemy at least as powerful as the reapers by itself. The entire Mass Effect galaxy combined was all but defeated in roughly a year.

Do you really want to elevate the Covenant to a Reaper-scale adversary level?

Still the UNSC got their asses kicked on almost every theater of war and even got lucky that 117 found said Halo ring in the first place and the realisation that they needed a human to interface with the forerunner tech prevented the Covenant to simply glass Earth in Halo 2. Correct me if I am wrong.

If you ignore the psionic potential of biotics and the impacts of the Mass Effect physics itself, then the UNSC would not just be on par, but vastly outnumbering.

As the whole Mass Effect universe is build on said Mass Effect physics, the Alliance would counter that. A Sentinel or Adept would fling a Spartan up in the Air and the Mass Driver Weaponry would shred the UNSC Vehicles and Gear into pieces.

The UNSC uses slipspace (hyperspace), thus being able to use outmaneuvering tactics, though the FTL-Drive works similar and is more easily to use.

It's apples and oranges in the first place but I'd bet my money on the Alliance.

UNSC definitely wins. People are always biased in favor of their own universe factions and that's ok because everyone has bias. But to be honest, UNSC stomps in space. Here are the reasons why:

1. UNSC ships have thicker armor

2. UNSC ships have nukes

3. Mass Effect ships will crumble from their firepower.

4. UNSC ships have point defense turrets

5. UNSC cruisers, frigates, carriers have MACs (magnetic acceleration cannons) which are capable of taking down Covenant ships that are much larger than Mass Effect vessels

6. Naval victories matter the most, like in the Human Covenant War. Even if the Alliance wins most of the ground battles, the UNSC navy will just nuke them and bombard them from above.

7. UNSC has Spartans who can sweep any infantry the Alliance can produce

Seriously, if the repeaers fought the UNSC navy, they'd crack them in half with MAC blasts!

@Tyboz9 The Covenant are way more powerful than the reapers. There is no comparison. I'm a massive Halo and Star Wars fan so I know those factions very well.

Repears are 2 kilometers long, in comparison Covenant Assault Carriers are over 5 kilometers long. Covenant Super Carriers are 32 kilometers long.

Halo Brute Chieftains and War Chieftians would bag any infantry the Repeaers had. Granted, the repears would be much better if they fought against Halo because then they could indoctrinate the brute and elite races. Sangheili Zealots, Ultras, Spec Ops forces, and the Silent Shadow would smoke them out. Hunters would run right through Repeaper brutes and they have awesome firepower (fuel rod cannons or wide laser beams).

@General B.H. I agree that halo is more powerful. My point is that since the UNSC was able to survive for a quarter of a century against an enemy that could probably take on the Reapers single-handedly they would have no trouble at all with a power that’s roughly equal technologically and MUCH lower in number. Honestly, they could probably fight the entire ME galaxy combined, maybe even with genophage free Krogan on their side.

^issue with that though as seen because of andromeda we know that shields in mass effect can withstand plasma weaponry so when every single person has a shield or barrier compared to the enemy it’s kinda More what’s the word difficult to kill a systems alliance marine vs a unsc marine there’s also a reason why I kept space combat out of these hypothetical situations halos space combat is far more evolved than mass effects but mass effects ground combat is far more superior

As tuff the unsc ship they are to big and bulk to useful In fight mass effect ships and reapers are made special alloy that citadels made out. So any cruiser or dreadnought would break the armor

Even though the Alliance certainly loses to the UNSC due to its inferior navy, let's compare their infantries.

Systems Alliance:

• 80% of its marines don't seem to have shields (basic infantry, rocket troopers, and snipers)

• The next 20% of its marines (the officers) have shields

• Special forces (N7s) have shields, a range of weapons/specializations, and great training

• Light vehicles seem to be MAKOs which seem to be made more for expedition than combat (but they are well rounded overall)

• Heavy vehicles appear to be Hammerheads which have heavy missile launchers that have longer range than the MAKOs and can hop around the battlefield

• Hammerheads are fast and can buzz around the battlefield making them one of the Alliance's best ground vehicles

• The big bulging grizzly tanks always get destroyed before we see them do anything, so I'd assume they're more like transports than actual combat vehicles

• Alliance forces beat UNSC troopers and mairnes pretty handily in direct fights due to having better weapons, using cover more, and more troops with shields

• But the minute most alliance troops face UNSC vehicles, ODST, Spartans (including Spartan 1s) they will lose they just don't have that flavor to them

UNSC:

• Has heavy combat mechs that can battle alongside troopers

• Has ODST who would beat most Alliance infantry even with worse gear and without shields (they've taken on tougher opponents anyway) due to superior skill

• Has Spartan 2s, 3s, and 4s. 2s with shields beat any Mass Effect infantry. 3s without shields, still beat any Mass Effect infantry but can lose during certain circumstances after inflicting heavy casualties

• Warthogs (standard, guass turret, and rocket all lose tbh) lose to MAKOs (MAKOs have more protection, missiles, and machine gun turrets)

• Scorpions beat MAKOs due to range but lose to Hammerheads due to them having superior movement

• Hammerheads lose to Grizzlies which have superior firepower

• Mammoths stomp Alliance vehicles due to superior size, firepower, and mac guns

• Mantis beats Makos and Hammerheads

• N7s would struggle against ODST but win due to having shields

• Spartan IVs have shields, super strength, super speed, greater durability, and probably equal N7s in skill despite having inferior skill to Spartan IIs and IIIs who were trained from childhood. Spartan IVs will beat all Alliance Infantry

• UNSC will reverse engineer Alliance tech, which would be even easier to do than against the Covenant and it's a thing they do

• UNSC AI is extremely advanced

• UNSC has other vehicles (blisterbacks, kodiaks, wolverines, and vultures) that would be a major problem for Alliance vehicles

• UNSC marines and troops can still beat Alliance troops using guerrilla warfare, something the Alliance is no longer to but the UNSC knows well due to fighting the Covenant

• UNSC troops have more experience due to fighting a longer war and superior tactics due to fighting a technologically, physically, and numerically superior foe and surviving