If they're able to build a time machine, the very fabric of spacetime will be torn to shreds!!
AIR
Aerial battles would be difficult to determine the winner. Sure, Broadswords would annihilate anything we have today, but Pelicans do not have energy shields and the charge time on the laser could bog their performance down. A Pelican versus an Apache helicopter would be an almost even match.
VEHICLES
The Scorpion would be toast, and every type of Warthog except the regular Warthog would be a threat if they aren't taken out quickly. But again, the Marines on these Warthogs are not well-protected, and a stray round to their faces will take them out. The recoil on the heavier Warthog weaponry could be problematic as well, since it could tip over the vehicle unless they're balanced properly. We must also consider that the UNSC cannot train Marines to drive to save their lives, and they would probably drive over a cliff or crash into a wall.
INFANTRY
Modern forces wouldn't even have to do anything. Marines are notorious team-killers in Halo 4, are too busy shooting corpses, or hardly put up a fight.
SPECIAL FORCES
We're doomed.
This argument is based on my observations in-game and of course make fun of how bad UNSC personnel are in Halo 4.