I wonder why “A Nation Divided” is in this headline instead of “A Divided Nation”. To me, from how I am taught, isn’t an adjective supposed to go before the noun? I am not a native speaker.
It’s an allusion to a very famous speech by Abraham Lincoln, which included the line:
A house divided against itself cannot stand. I believe this government cannot endure, permanently, half slave and half free.
As you see, in Lincoln’s speech divided is accompanied by a following complement, against itself. A modifier which itself has a following modifier or complement cannot be placed in front of the head noun, but must be placed after it.