Enclave had a oil refinery. SO yeah it would make sense.
What if that was the last oil refinery in existance? Gone now
There was a war because the world ran out of oil, and USA claimed the last oil in the world for their own taking, not sharing with the rest of the world. Then they also developed atomic technology, like cars powered by nuclear reactors and batteries instead of oil, while Europe was stuck with regular cars, no gasoline and bitter infighting for years like they reverted back to the dark ages again!
I just found out that the NCR cavalry (which someone pointed out was with vehicles and [censored]) consists of the NCR heavy troopers. Makes sense, since they are kinda tanks with their little power armor and [censored].
You mean like "mechanized infantry"? Cavalry ain't really cavalry if they are heavy troopers on foot, not even if you can compare them to being tanks which ain't cavalry either, that's closer to mechanized infantry too if they'd ride vehicles to combat. NCR Cavalry would be http://images.wikia.com/fallout/images/9/98/FNVAR_Horses.jpg and http://savagebeatings.deviantart.com/gallery/34104752#/d4b7jwp.