Reading about the complexities of Europe has made me wonder about when American (internal) borders were less porous. Also, were the borders between the 13 colonies completely open? Or did they at least have had customs houses to collect trade tariffs?
I can't speak to the early cases of the colonies, but today there are still border patrol stations within the United States. The one I have been through several times is the San Clemente station, 67 miles from Mexico on Interstate 5, in Marine Corps base Camp Pendleton:
The San Clemente Border Patrol Station maintains a full-time traffic checkpoint on the northbound lanes of Interstate 5. It is one of seven stations in the San Diego Border Patrol Sector and operations one of four checkpoints. Checkpoint activities are directed against the smuggling of illegal aliens and narcotics away from the border area.
Article 1, Section 8 of the Constitution states:
The Congress shall have Power To lay and collect Taxes, Duties, Imposts and Excises, to pay the Debts and provide for the common Defence and general Welfare of the United States; but all Duties, Imposts and Excises shall be uniform throughout the United States;
Article 1, Section 9 states:
No Tax or Duty shall be laid on Articles exported from any State.
The borders of the United States have been porous for most of our history, however during much of the twentieth century the immigration laws were more strictly enforced. This began to change about the mid-1960's.