If Rome at the height of it's power is with Julius Caesar and the 10th legion (Legion X Equestris) leading the show, then Sauron's army is absolutely fucked.

The legion would retake osgiliath, shooting down Nazgul and their beasts with ballistae as needed. From there, the Roman engineers would build a bridge across the river capable of supporting their whole army as well as their siege equipment. They would then build miles of trenches, walls, towers and other defenses at the perimeter of the black gate (everything I've detailed so far is what Caesar's legion did in the Garlic conquest/siege of Alesia). All of this they could accomplish in under a fortnite.

Each orc soldier is likely comparable to a Celtic warrior, whom Caesar and his men were able to defeat pretty handily, even significantly outnumbered. 500k orcs vs a fortified Caesar, his 40K legionaries, and the rest of the combined forces of middle earth? EZ. They could just walk right into Mordor with the ring after that.

Frodo and Sam would still need to carry the sucker into the volcano but they'd have saved themselves taking the long way.