Some say Afghanistan.
Ever heard this quote before ? For every soldier who has paid the ultimate price in the Graveyard of Empires, You are not forgotten! What do you think the Graveyard of Empires is?
I'd never heard Afghanistan called that till you asked the question, Rooster. There's a very interesting Quora article at the link below.
The Afghans are ferocious people and their land is a fortress that has denied all would-be conquerors from Alexander to the USSR (both points made in the link article, which is worth reading).
Never heard it, but I would think it is just a way of saying those that died for their country or kingdom.