I am on mobile/phone so it will be aggravating to type excessively so I will type some of my thoughts out in bullet points:
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Assimilation:
@da Racontor provides a sadly true and ugly side of it and should not be discounted. But I will also say the USA was formed right when nationalism as we know it came to the fore, the Anglos were a majority (arguably supermajority at around 75% of ancestry), and assimilation to the founding Anglo ethnolinguistic culture seemed a winning idea in conjunction with:
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Ideals: modern-day memes and politics aside, the USA was an attempt to provide liberty and prosperity to the common man, and that did expand over the decades and centuries as non-propertied white men gained the franchise, then slaves were freed and gained it (even accounting they had to fight Jim Crow for a century), then women, etc. And for much of history most people could make a decent living in cities or strike out to being a farm/homeowner inland, doing better than their ancestors did.
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Unionism as a specific ideal: the colonies could “hang together or separately” as ole’ B-Frank stated, and then you had the Civil War fought to preserve the Union as
@Hugh Third of Five pointed out alongside emancipation. That is a POWERFUL mythos to be founded on, then protect, not even a full century later.
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Federalism: despite the federal government growing more powerful over time, the state governments had much local power and still a large amount today. This lets localisms thrive while remaining unquestionably “American” in their and others’ eyes and work to local needs as they see fit.
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Geography: east of the Rockies is flat land outside the easy-enough-to-cross Appalachians, then the Intracoastal Waterway of the Mississippi watershed/Great Lakes/Atlantic Ocean/Gulf of Mexico. West of the Rockies is South Pass - one of the greatest mountain passes on Earth - opening right into the Snake River, then that onto the navigable Columbia, that in turn flowing into the Pacific Ocean. Outside of sheer distance, geography is so damned blessed east of the Continental Divide
and then God stepped in with a goddamn amazing mountain pass and navigable river straight to the Pacific, I wonder if he was playing America in a Civilization match and created Earth as we know it so he could dominate the “game”. This geography ALSO allows incredible ease of trade and travel, which in turn ensures ethnolinguistic and cultural connection and continuity.
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Technology: …and all that even BEFORE
@SWS rightly pointed out how the railroad and telegraph worked to make even the Intracoastal Waterway (
almost, mind you) obsolete over time! These came in conjunction as Americans were still settling the majority of their territory, once again linking everyone easy and ensuring a monoculture.
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Population numbers and density: Americans were so numerous and had such a high birth rate not only did they sweep away dozens of weakened (via disease, intratribal warfare, Euro warfare) Amerindian tribes and nations, they completely overwhelmed what very few “rival” European colonists did exist in lands they took over be it Tejanos, Californios, and Neomexicanos in the Mexican Cession, Hudson Bay Company’s British traders in coastal Washington state, and even acculturating the Gallic Creoles and Cajuns in Acadiana. They were buttressed by the federal government’s support, technology linking them back east and letting them spread out fast, and the assimilation point above.
John Adams (born 1735) and Thomas Jefferson (born 1743) were born when the only Anglos on North America were in the Thirteen Colonies and Newfoundland. By their deaths in 1826 Anglos had well-settled Missouri and were snaking into Manitoba, Arkansas, and Texas. By the time their grandchildren’s generation had been established in adulthood and the frontier closed in 1890, Anglos dominated all the USA and Canada outside Quebec and the southernmost portions of the Mexican Cession. In the course of only over a century and a half and two full generations, Americans reached the Pacific. With how short 160 years is in history that alone ensures a sort-of monoculture and living memory from grandparent to grandchild from coast to coast that could take root. Whatever else one can say if it good or ill, it really is impressive.
