So I've been reading the books on Gears, the first got into depth about the Pendulum Wars. The COG vs. UIR. Just wondering, what happened to the UIR? Obviously the COG stealing the Hammer made them surrender, but what happened to them since E-Day? Did the COG assimilate them or have the UIR nations been completly destroyed by the Locust and COG-Hammer deployment?

i honestly don't think the story in gears needs backgroundounf or even any depth but i guess whatever floats your boat.

Hey man, I just started reading the books, and have been wondering the same thing. I just started Jacinto's Remnant. Aspho Fields was great though. It's nice to have other Gears stuff to absorb.
 
I figure the UIR were wiped out by either the COG or by the Locust's original attack. The story explains that the human race is down to millions, so it's possible that so many people have died that only COG are left. Although the Pesang troops aren't COG, so it's entirely possible they just don't focus on the UIR if they do still exist. 
 
Glad to find someone else who's reading them though!

Perhaps they dissolved in to the COG?

There's no mention in the books, and plenty of details regarding the war before E-day, but it's as possible as anything else!

The war went on for a little while after the COG stole the hammer but eventually most UIR nation dissolved and came under COG rule but some refused to surrender and stayed independent . 

Read the second one. The UIR makes a guest appearance.