Background: The US Constitution was a document that united thirteen different countries into a single one. Previously, the thirteen colonies turned states banded together to fight the British but the first American constitution, the Articles of Confederation, wasn't really a constitution; it was a treaty establishing an organisation to coordinate them. That organisation was the "United States in Congress Assembled", which functioned more as a diplomatic conference than a legislature, hence the term "congress", also used in terms like "Congress of Vienna". This organisation had no executive or judicial power. Compliance with its decrees was essentially voluntary. The Declaration of Independence is titled "The unanimous Declaration of the thirteen united States of America...". Note that "united" is not capitalised, because it is not part of a proper noun ("Declaration" and "States" are capitalised following an old English convention that capitlaises every noun). They are thirteen independent states that happen to be united for the purpose of this declaration.

That being said, the Philadelphia Convention that wrote the US Constitution did so clandestinely. The Convention was called to suggest changes to the Articles of Confederation, not write a new constitution and establish a new federal state. To keep their work secret, they shut the windows to the State House where they worked despite it being in the middle of summer in a time before air conditioning.

As the thirteen states were, at the time, functionally independent countries, they could not be forced to abide by a constitution that they did not agree to. Many southern states had agricultural economies that relied on slave labour. Slaves were valuable property to those who owned them and the men who did were usually rich, powerful, and influential. It's understandable why they were not willing to give that up, even if it meant defying the idea of "liberty" that they fought for.

This was a cause of much squabbling in the Philadelphia Convention between delegates from northern states, which were not reliant on slave labour, and delegates from southern ones, which were. This resulted in the infamous three-fifths compromise, allocating representation in the US House of Representatives according to population, counted as the number of free people plus three-fifths of the number of slaves (the used the term "other persons" as a euphemism for this). Indigenous people were explicitly excluded from the count as well. Franchise was determined according to state law, which gave southern states the power to withhold suffrage from slaves.

This was considered agreeable to southern states and acceptable to northern states. Thus, when all thirteen states ratified the US Constitution, they permanently signed away their sovereignty and became a part of the United States of America, a new country.