There are few truly compulsory actions required of US citizens, but there are a great number of conditional requirements, and of course there are many criminal and civil laws governing daily life. Since you have stipulated land ownership in your scenario, this answer is in two sections.
Compulsory actions without any qualification:
- Birth. Registration of live birth is required by every state. Hospitals facilitate this, but the ultimate responsibility is with the parents to register births with the county court.
- Military service. All male citizens must register with the Selective Service from age 18. If drafted, men must serve.
- Education. Some form of schooling is required in every state. Vaccination is usually a requirement to attend public and private school, but some states require child vaccination universally. In some states, vaccination is optional for the home-schooled.
- Taxes. Income tax filings are compulsory only if taxes are owed. (This means that registering for a Tax ID or Social Security number is not technically required unless your income rises to a level that causes you to owe income tax to the state or federal government.) Sales taxes are normally paid by sellers, but if not collected by sellers, must be paid by buyers.
- The census. Federal law requires every citizen to respond to the census, though in practice, many do not respond and suffer no consequence.
- Submission to courts. Any court summons or court order must be followed including for jury duty, although registering for jury duty itself is not required by every state.
- Death. Under state laws, deaths must be reported by whoever is in attendance or whoever discovers a person has died.
Conditional on land ownership:
- Accurate records of land transactions must be recorded with the county, and recording taxes and fees paid.
- Each time land is platted (split or combined with other land plats) there are requirements for surveying and recording the land boundary, and typically a street address is assigned by the US Postal Service.
- Property taxes must be paid in states that collect them.
- Numerous laws specify what may be built on land according to zoning, how structures must be built and maintained, how land is to be used, and the permitting process in each county.
I exclude the bulk of criminal and civil laws from this answer, even though every citizen must be subject to the sovereign law, since it is generally possible to avoid running afoul of these laws by simply doing nothing and remaining on your own land minding your own business.
However, in your scenario you are basically imagining a commune. It won't be practical for the commune to exist on blank, undeveloped land without any economic activity. For a commune to remain completely isolated with no external trade would be extremely difficult in order to not revert to an extremely primitive lifestyle. And even hunting and fishing for subsistence has to be licensed and/or permitted in every state.
A 'rogue' subsistence farm of any size that operates completely without county oversight, once discovered, would be quickly shut down anywhere in the United States. Once your commune starts farming, or operating businesses on their land, building housing, drilling wells, buying and selling, etc. they will be active participants in the society of the county where they are, and it will not be possible to remain totally isolated. The 'minimum requirements' for existing and remaining in compliance with the laws will greatly increase, particularly if the commune operates an agri-business. For example, livestock has to be certified to be cared for and disease-free, dwellings and barns must be permitted and inspected, selling produce is regulated, and property taxes have to be paid annually in most states.
So the above list is basically a minimum list of legal requirements just for a US citizen to be born, keep breathing and die. How people live brings different legal requirements into their lives, but always many more than just this list.
