When and how did the United States of America cease to exist (as a sovereign country)?

Question

It is mentioned many times in the original Star Trek series as well as The Next Generation that the United States of America has not existed as a sovereign country for some time. It is, at most an administrative division of United Earth and/or the Federation, and may only be a colloquial geographic designation for certain Earth districts disconnected from any contemporary political realities.

In the Prime timeline of Star Trek, when and how did the USA cease to exist as a sovereign country? Did the USA collapse completely in or shortly after World War Three, or did it maintain a continuous existence until it joined United Earth sometime before Enterprise? Are there any resources that cover the specifics of the final fate of the USA or is it left completely ambiguous?

In the film Star Trek: First Contact, we catch a glimpse of one corner of America in 2063 as Zephram Cochrane prepares for his first warp flight, but the state of the government appears to be left intentionally ambiguous. Riker observes that, "...most of the major cities have been destroyed, very few governments left, 600 million dead. No resistance.", implying that at least some governments may have survived. It is thus unclear whether the USA has completely ceased to exist in any meaningful sense in 2063 or whether it has simply been weakened to the point where it is unable to maintain a consistent rule of law in rural Montana.

In the TNG episode The Royale, we learn that the USA and its NASA space agency apparently existed until at least 2079 (with a 52 state flag). The episode does not tell us whether the USA collapsed, joined United Earth, or simply added another state in 2079. At the beginning of Enterprise in 2151, we learn that the entire planet has already been united under United Earth and there are no more individual countries.

To be clear, I'm not asking whether the USA still exists as a sovereign country by the time of Kirk and Picard. The overwhelming, consistent evidence in Star Trek indicates that it does not. My question is on whether the "nuts and bolts" of its collapse, annexation, or reformation has been covered in any Star Trek medium or source.

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In response to a comment by NKCampbell, I am aware that cities such as San Francisco are shown to still exist in the 23rd and 24th centuries, but that is what I not asking about. Many cities that exist in the world today have survived under multiple governments. All of the major cities of the Republic of Texas continue to exist in 2022 even though the Republic of Texas was no longer a sovereign country after 1846.

In response to Paulie_D, I am asking about existence as a sovereign country rather than existence as a political subdivision or geographical designation. Many formerly sovereign countries (e.g. Texas, Hawaii, Scotland, Newfoundland) have lost or given up their sovereignty but still exist as administrative subdivisions of other countries. This may well have been the fate of the USA in the Star Trek universe. My question is about that specifically. For example, something like, "According to this specific episode of Lower Decks (cite), the 54-state USA requested and was granted annexation into the Greater Canadian Empire in the spring of 2102, becoming the Yankee Imperial District alongside the Old Canada District and the various formerly sovereign districts of Latin America and the Caribbean. The Empire then merged with the Afro-European Conglomerate and the Pan-Pacific Consodality in 2135 to form United Earth." would be a great answer.

Answer 1

Per Star Trek: Federation - The First 150 Years, the country known as the United States of America ceased to exist in 1994, shortly after the start of the Eugenics Wars. After multiple nuclear attacks devastated its major cities, the US was eventually merged into a conglomerate consisting of Canada, the US, Mexico and the entirety of South America and renamed 'The American Empire'.

North, Central, and South America became the American Empire under the dictator Asahf Ferris; Australia, New Zealand, and the Pacific Islands fell under the rule of Bernard Maltuvis; Europe was ruled by John Ericssen; and Asia through the Middle East, dubbed the Eastern Coalition, was under the control of Khan Noonien Singh.

Even after the Supermen were expunged and genetic augmentation made illegal, America seems to continued to consist of all of the Americas.

At the time of Cochrane’s birth, civilization continued to deteriorate, and, with a few blips of hope, careened head-long into barbarism. In 2037, NASA was re-established and had launched the Charybdis, a manned extra-solar probe. But only a few years later the forces of anti-intellectualism took root in the former United States (now part of a failing democracy that included Central and South America) and in 2044, responding to the public unrest, the government ordered a purge of academia.

The precise basis on which nations exist within the United Earth formed in 2150 isn't especially clear, but for our purposes we can assume that it never became a sovereign nation again after 1994.