
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.

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.
The adjective preceding the noun is a general rule of English; however, it is not the only location. It is possible to emphasize (some) adjectives by placing the after the noun. Example: "A tiger, large and menacing, crept through the jungle." The alternate sentence, "A large and menacing tiger crept through the jungle," doesn't emphasize the qualities of being large or menacing.
Those two headlines give me different expectations for what the content will be about.
A Nation Divided I expect to tell me about the process leading to the division, but not necessarily what the end result will look like.
A Divided Nation I expect to tell me about the state of this divided nation, but not necessarily how it got there.
It is of course possible that the content will cover both the process and the end result, since they are obviously related, in which case I would find either headline to be suitable.
There are some adjectives that follow the noun in English. But in this case, haven't we just got a hidden relative clause. "A nation [that is] divided." Similarly with Lincoln "A house [that is] divided against itself cannot stand."
Since 'Divided' can either be a verb in past participle form (as used in passive clauses) or an adjective (quote from @bdsl - thanks for clarifying the distinction for me in the comments below), my impression is that "A Nation Divided", with the emphasis on the "Divided", might be referring to the verb form - the action of having been divided - more of an emphasis on the action and less on the object of the division - and with connotations of a temporary state or divided with regards to a certain issue or a division that has happened recently, whereas "A Divided Nation" sounds like the adjective form and would seem to be projecting the idea of some kind of permanence, irreconcilability or essence.
I am no expert on English, but here are my (informal) thoughts:
It is typical of the media or the news agencies to twist sentences to get catchy titles.
If you want a news article, then the second thing makes more sense. You are willing to report an event, not describe the nation.
I think the answer is that "A Nation Divided" uses the past participle form of the verb "to divide," implying an actor or force that is responsible for the division (e.g. "A Nation Divided by Ideology"). This grammar form is somewhat archaic, and can certainly be found in the King James Bible as well as Shakespeare ("A house divided"). Converting "Divided" to an adjective placed in front of "Nation" reduces the amount and type of information the headline conveys.
This is also a great example that news headlines aren't really governed by the same grammatical rules as is typical written English.