The US is officially a constitutional democratic republic. It has a constitution that specifies form, stabilizes, and limits the government. It is democratic because the populace elects representatives instead of them being appointed, and it is a republic because it runs under assemblies of representatives.
Our congress has two assemblies of representatives. One is the House of Representatives who represent the populace in general. Each state sends a number of representatives according to its population.
The other assembly is the Senate. Each state sends two senators now elected by popular vote but originally they were appointed by each state assembly. They represent the states and the various cultures dominant in those states.
Originally, and still to some extent, various cultures from various places mostly in Britain and Europe established colonies and territories of their own. The conflicts between them would have been much more severe if the Senate didn't give each a chance to influence government. The most populated cities would have taken control. Cultural conflicts beyond the universal one between rural and urban cultures that causes most civil wars and revolutions would have been much more common here.
Below are some details about how we arrived at changing the US government from a confederation to a federation. The seceding states in the 1860s went back to a confederacy until the federal union forced them back into the federation of the United States. Our Civil War began when the Union prepared for blockading southern ports because they refused the tariffs and seceded.
The structure and principles of government here in the US are all wrapped around the very fierce battle between Federalist Party bankers and merchants and anti-Federalist farmers and tradesmen. The latter soon formed the Democratic-Republican Party ultimately known as the Democratic Party (today's Republican Party wasn't formed until the latter 1840s), but with true liberal policies not the socialist and progressives ones of today. The anti-Federalists strongly opposed many Federalist policies and did not trust the new constitution the Federalists wanted, so they demanded our Bill of Rights to protect themselves (and us) against domination from the Federalists.
The primary reason the Federalist bankers and merchants called for a convention to alter the existing (then) constitution (the Articles of Confederation) was because that constitution allowed states to shut out businesses from other states. Those articles formed a "confederation" of independent states. Those were like independent countries aligned together without the overriding federal authority we have today. The Federalists wanted changes that allowed them to move their businesses freely between states.
The Federalists also wanted to form "well regulated [meaning fully trained and equipped] select corps of the young and ardent" (see Federalist Paper No. 29) formal militia units drawn from within the already existing (since the beginning of the colonies' formations) militia of all able-bodied male citizens. The new formal militia of disciplined (and indoctrinated) young men (good for dominating states and citizens) was wanted by them to put down rebellions like Shays' Rebellion a year before had been against Massachusetts banks. That rebellion was made from (of course) many members of the normal citizens' militia.
The arguments between Federalists and anti-Federalists, under many compromises hopefully students are still taught, resulted in the new Constitution we have today. Fallout from the conflict between the two factions eventually cost Federalist leader Alexander Hamilton's life from a dueling wound, and culminated in our Civil War decades later.
That war did not result from the hot issue of slavery, as both of us were taught. Secession was inspired by oppressive tariffs on southeastern Democratic state's lucrative foreign trade. Northern Republican Party (they had succeeded the defunct Federalist Party) controlled states were thrown into deep economic depression by huge hordes of my starving Irish relatives undercutting wages, just as today's on-coming depression is caused by horrendously huge hordes of overpopulating Asian excess labor undercutting wages
The tariffs were needed to bail out the suffering north, as well as a whip for the English descendant merchant controlled north to beat the Scots-Irish descendant farmer controlled south for perpetually resisting their domination here like they did in Great Britain.
For decades I studied philosophies, cultures, and social institutions. I began that because of confusion resulting from my military experience under the shadow of neo-Marxist anti-military and anti-capitalism indoctrination in the universities.
I continue a forty year quest wading through the huge pile of stinking crap a wide variety of bigots piled on top of truth hiding it from nearly everyone's view. The pile was made by blaming people they don't like while excusing people they do like regardless of where the fault really lies.