I've seen this debate a few times, but none since Halo 4, which changes the playing field significantly.
So, the scenario is, a UNSC Invasion Fleet with four and a half million Marines and all the relevant vehicles, air support, etcetera, is invading the USA. The USA and any nations willing to help it have to defeat the invasion force to win, the UNSC has to occupy the entire US and force any of it's allies to surrender. Unfortunately for the UNSC, a fluctuation in the time-space continuum resulted in the UNSC Fleet having no ammunition for it's spaceships, ruling out orbital bombardment.
Air
I think that in the air, the UNSC would prove victorious. The Broadsword is equipped with energy shields, which means it would be able to resist modern weapons, and it's firepower is impressive enough that it could shoot down any enemies.
The Pelican in Halo 4 has lasers, which means any helicopter challenging it would be decimated. We haven't seen the Hornet or Falcon yet, but they probably have lasers or shields.
Armour
This is a tricky one, but I think the modern forces have the edge, mainly because the Scorpion and Warthog aren't very well designed from a practical standpoint.
The Mantis is powerful, but I doubt it could take on a modern tank column. As for the Mongoose... Well, it's a non-factor, really.
Infantry
In terms of infantry, the modern forces would have the edge, simply because of the MA5C's awful range, and the power of modern weapons.
Special Forces
The UNSC has SPARTAN-IVs, so in this section they would absolutely dominate. ODSTs are also superior to modern special forces, and they can be dropped from orbit, giving them practically infinite access to target locations.
What do you all think?