The US Civil War is seen as an act of nation building, which is why in the documentary movie “National Treasure: Book of Secrets” Nick Cage says that before the Civil War people said ‘the United States are’ and after ‘the United States is’. More seriously, at least in liberal theory the South was a nation. From a paid article by Big Serge, someone i would call a well read liberal:

The US Civil War was, as I would argue, the single most consequential act of empire building in modern history. The simple fact was that the Confederate South was a nation, or at least was in the process of becoming one, with a wealthy agrarian economy, peculiar social forms, and a patrician leadership caste that was largely alien to the industrial, urban north. Southerners affirmed their membership in this emergent nation with exceptionally high levels of military participation, the willingness to endure extreme privation, and a new schema of southern symbols and hagiography. This emerging southern nation was strangled in its cradle by the powerful north and then re-integrated into the Union in a complex political settlement - the cost of which was abandoning southern blacks to a postwar racial caste system.

More materially, the capital used to jumpstart the London Stock Exchange ultimately originated in the primitive accumulation of chattel slavery in the US South. The low cost of the cotton made mills in Liverpool profitable, and that allowed for finance to emerge. The Confederacy thought during the war that Britain would bail them out because of their economic inter-linkages. Now, Britain was actually engaged in imperial expansion in Egypt and India for more cotton under their control, and didn’t really mind. But for over 200 years, slaves worked in particular parts of North America to produce raw materials and those were processed in England.

By contrast, the Union was a shipbuilding pit stop in colonial times. Lumber, pitch, hemp, shipyards, and rum distillation were what the Empire wanted. The various English colonies in North America had different laws and different economic purposes. Famously, before the war, the South preferred to send cotton to England instead of mills in the North, effectively subsidizing their competitors. The rules and structure of settling westward and stealing more native land were also bound by the competition of northern yeomen farmers and proto-industry vs southern plantation owners and highly militarized lower classes.

It might be wrong to technically call them a nation, but the alternative would be something like “for 100 some years, half the country fought tooth and nail for how much they loved slavery”. Better PR to call them a totally separate enemy. And since this is the Union that made slavery legal in prisons where settler citizens can’t see instead of banning it, PR counts for something.