If America was ever forced to break apart, which states would join together first?

First things first: despite our differences, I think that the vast majority of Americans recognize that we’re far stronger and better off together than apart, either as individual states or as groups.

Second, as Colin Woodward points out in his brilliant American Nations, even individual states show vast differences culturally within themselves. He suggests the following map as a more accurate breakdown of the various “nations” that make up the United States:

 

Please bear in mind that all of the below descriptions are a brutal oversimplification of Woodard's masterful work, which also includes nations that include "territory" in Canada & Mexico. Read the book!

Yankeedom: This nation began with the Puritan settlers of Massachusetts. Overall, they are more in favor of strong central government and governmental intervention to make the world a "better place" - in their image, naturally. They tend to value education and communal decision making.

Deep South: At its heart, this nation was started with the goal of creating a mainland version of the Carribean sugar plantations, but focused on cotton rather than sugar cane. They tend to see themselves as a recreation of classical Greco-Roman society: individual freedom for some, slavery for others, and free markets for all.

New Netherlands: this is New York City and its direct environs. This was a North American recreation of Amsterdam, the most capitalist & muticultural European city of its day. To this day, greater New York City has a similar, hyper-capitalist, tolerant and diverse culture.

Tidewater: This nation was founded by Cavaliers (Royalists from the Engish Civil War), and produced many of the Founding Fathers.

Greater Appalachia: This region was settled by people escaping the violent border region of northern England and southern Scotland, as well as Scots-Irish. When they arrived in America, one of their primary goals was to get as far away as possible from the east cost cities like Philadelphia & New York City as possible, so they chose to settle in the Appalacian mountain region. This tends to be the region focused on what we might call "libertarian" values, as well as the one that often produces decorated US soldiers and generals.

Midlands: This nation was founded by Quakers, followed by the so-called "Pennsylvania Dutch" (actually Deutch, meaning German), & spreads from Philadelphia out to much of the Midwest and into greater Toronto. As such, the region tends to be tolerant, somewhat pacifist, and diverse. This might be seen as the most uniquely "American" of the nations.

New France: As you might guess, this nation is almost exclusively limited to southern Louisiana in the US (fun fact: while most states base their common law exclusely on English common law, Louisiana bases its on both English & French common law), while it also includes French Canada.

El Norte: This is actually the oldest "nation" in North America, and includes northern Mexico as well as much of Texas, New Mexico, Arizona, California, & parts of Colorado.

Far West: Despite its name the Far West is not the west coast of North America. Rather, it is the "inland" sections of California almost as far east as the Mississipi River, plus Alaska. It tends to be suspicious of governmental and business "imperialism" from other nations.

Left Coast: This is an odd combination of utopian idealism that wouldn't feel out of place with libertarian leanings that remind one of either Far West or even Greater Appalachia. It basically runs up the entire west coast of the US, plus Hawaii.

First Nations: This is a native American nation, largely based in northern Canada, plus including Alaska. As a whole, it does not share any European characteristics with the other nations.

Again, this is a brutal oversimplification of Woodard's work, and not at all meant to be an adequate summary of it.

US history can be seen as one of alliances between the various nations. For instance, during relative peacetime, a "Northern Alliance" of Yankeedom, New Netherland, and the Left Coast vs. a "Southern Alliance" of the Deep South, Greater Appalacia, and Tidewater. The other nations act as "swing nations" whose temporary alignment with a larger alliance swing the balance of power in the nation as a whole. During major crises, larger groupings of the nations can come together and determine a national response. During WWII, for instance, every nation was aligned, but all had completely different reasons for involvement.

I can't hope to summarize such an impressive book in a short Quora post so, again, read the book!

https://colinwoodard.com/books/american-nations/