More of a follow-up question. I was taught that a portion of the difference in culture between the North and South originated from the Northern states becoming somewhat more of an industrial society where the South remained agricultural. If this is indeed the case what were the factors leading up to this shift? I know that climate would be partially responsible but I am wondering if there was more to it.

I'm only an armchair historian and waited a day for a real historian to help you. Given that isn’t the case, I’ll try to help.

they [the slave-owning clases] painted the northern policies and actions as being about attacking the southern way of life, not slavery.

That isn’t really the heart of what happened. There has been historical "Lost Cause" revisionism where the cause was painted in the light of preserving Southern Culture. If you look at the words of the Fire-Eaters and the speeches of famous Confederate politicians, they were utterly focused on the topic of the need to preserve and expand slavery. Just read any of their speeches and newspaper articles. There was no subtlety to it. You absolutely will find calls for a war to preserve slavery. You really won’t find much in the way of cultural arguments (e.g. Southern people are more hospitable and that will be wiped out by Northerners.)

why and how did the southern identity come to be? Why was it seen as so thoroughly different from the North?

This is a big and complex question with many possible answers. The simplest and answer is “slavery.” “The southern way of life” and what made it different from the Northern way of life was all about Slavery. In the North, the vast majority of families lived in working class conditions but lived in an upper middle class sort of lifestyle. They worked to put food on their table. People could afford to buy things. So, they could furnish their houses and buy items to increase their material quality of life. It was a land of opportunity attracting a lot of immigration into new territories and thus wealth was fairly evenly distributed.

In the South, slavery disrupted the whole economic system and it has a massive impact on the entire society. The book “Cotton Kingdom” by Fredrick Olmstead was a record of what day to day Southern Culture was, how deeply engrained slavery was in that culture, and how slavery affected everything. In summary, there was a very wealthy class of plantation slave owners (about 5% to 10%) who owned most of the slaves, held almost all the wealth, and basically ran the whole show in the South. The second group was the slaves. The last group was poor whites who struggled to get ahead against a system stacked against them. As the plantation owners did not pay for public schooling, this class of individuals often went without much of an education and were often illiterate. They were squeezed by both sides and basically just worked enough to put food on their tables. The way these three classes lived their lives and interacted for almost 200 years was core to the Southern Culture. The Fire Eaters mainly cared about the impact of slavery on the rich plantation owners and the war was justified on that. I think some people like to reminisce of the lifestyle and culture of the rich and famous plantation owners and imagine that was the culture of the entire South but that is forgetting the other 90 to 95%.

And why weren't there any further geographical/cultural subdivisions of such importance in pre-civil war U.S.? .

I’d say there was. There were old Southern States like Virginia and Maryland where the land was used up and people were moving onto other activities than slave based agriculture. There were new states in the West (e.g. Kansas and Missouri) where farming was difficult and slavery was a huge bloody question. There were very slave intensive states like South Carolina, Mississippi, and Alabama which were passionately in favour of slavery and war if required. Other slave states (i.e. the border states) eventually sided with the Union and ultimately remained neutral when war came.

But in a way, there wasn’t a North and South. Many people moved between states. Beyond regions, people varied by religion, ethnic background, wealth, education, and their moral concept of what was good and bad. As a result, there were individuals who lived in the North and supported the South and the institution of slavery and there were men in the South who were so passionately against it that they eventually joined the Union army. Even families were divided. Lincoln’s own brother in law fought for the Confederacy. I know you asked about culture rather than War but I do find it insightful that when it came times to choose sides, things were not so clear.

But to ultimately answer your main question of why did poor Southerners picked up arms and fought for the slave owning rich plantation owners, I’d say the answer is that they were lied to and brainwashed. The newspapers in the South told ridiculous lies. They lied about the intentions of Lincoln to eliminate slavery. The lied about the benefits of slavery. They lied about how easy a war would be. They lied that God wants slavery to exist on this earth. Simply, the answer to your main question is the word propaganda rather than an explanation of cultural differences. Men can easily be stirred up to fight wars.

In terms of references, I’ve read dozens of articles written by Fire Eaters and don’t see the point in referencing one of them. They are all kind of the same thing. They can be easily found via a quick google search. I do recommend the book mentioned above by Olmstead.