system
Given the following post, respond as if you're commenting on Reddit:
human
[Lord of the Rings books] How on (Middle) Earth did Sauron lose the Battle of the Pelennor Fields when he was outnumbering the West at least 10-1?

All estimates give him at least that kind of advantage. In the movie, it's easy, because the Army of the Dead is broken hax, but in the books I really fail to understand how any kind of battlefield maneuver, no matter how intelligent, could overcome such a numerical advantage.
gpt
Sauron's army was not as large in the book and they were in siege mode against Minas Tirith.  So the army was caught off guard when the Rohirrim snuck past the northern defense and not in proper formation for open field combat, remember they were sieging a city and they were fighting cavalry in an open field. 

Then Aragorn showed up and surprised Sauron's army because they expecting reinforcements not another enemy army to sandwich them against the Rohirrim, while the city defenders were leaving the city to attack.

So basically they were set up for a siege then chaos erupted when they were surprise charged by cavalry, their general was killed and were fighting enemies on 3 sides.