If you mean the civil war, that wasn't to "keep them together": it was one side forcing the other to get back in line and give up its slaves. It was not a conflict where two somewhat equal partners work out their difficulties, it was a war where the central government fought down an insurrection and during witch the south got utterly devastated and was under occupation for 10 years.

It's also the urban centers from the rest of some states. For instance I live in MN, the city center is very blue and the rest of the state is not.

Money/economy was certainly a big reason.

Back in 1860s, cotton and other plantation heavy industries where very lucrative, the southern states were very wealthy enough to be able to field a big army for years, while they lacked strong industry to support an extended campaign.

Also manifest destiny and especially Monroe doctorine all pre dated the civil war, there was certainly strong idealogoical belief on who the continent was for.

I'd say that the pressure for fission becomes higher as the scope of federal government increases. Using spending as an approximation for control:

https://fred.stlouisfed.org/series/FYONGDA188S

WWII being an outlier of course. I'd like to have the numbers for the 19th C.

There was a time when a state/county/municipality could largely ignore central government diktat. With centralization there'll be a tendency for a single dominant group to get into everyone's business more and more. This can happen via statute, spending, or even lack of law enforcement in desired behaviors.

..and of course it tends to split urban vs. rural, Anglosphere vs. everyone else. The wish or need for central control varies quite a lot between the groups.

Lincoln's second inaugural address[0] spells out why in some detail.

0. https://www.ourdocuments.gov/doc.php?flash=false&doc=38&page...