Viper Skills:

Haha, this is a pointless question. The United States would shred the UNSC apart. Honestly, for being 500 years in the future, they have terrible weapons. Play Halo 3 to find this out. A Battle Rifle has the range of a bow and arrow. Modern assault rifles out-perform Halo ones. Which means that infantry goes to the USA.

Vehicles...

US tanks have no problem killing an entire squad in one shot. More powerful tanks. Anti-air units like the Wolverine have a grenade launcher as their main defense. It's laughable. USA wins vehicles.

Air...

Haha... F-18 vs. Hornet, you decide...

Space battles? UNSC finally wins! Oh snap!

Two things.

1. BALANCE FOR GAMEPLAY!!!!

2. SPARTANS!!!!

Need I make them in bold? THE UNITED STATES WOULD BE UTTERLY DESTROYED! Felix basically ripped everything I was gonna say out of my mouth, so I'll leave it to him.

The only true threat there is would be Spartans. But quite honestly, with the Marines in the USA using weapons that make the MA5B Assault Rifle and BR55 look like peashooters, a Spartan would be dropped pretty quickly, even to a single squad of Marines.

Uh, no. It would take the entire United States Marine Corps to take down all 33 Spartans.