Final answer:
The early 1800s belief that a state could refuse to enforce a federal law was known as nullification, based on the idea of state sovereignty and the rights of individual states. This belief was particularly associated with the issue of slavery and the tension between the federal government and southern states.
Step-by-step explanation:
The belief that a state could refuse to enforce a federal law in the early 1800s was known as nullification. This belief was based on the idea of state sovereignty and the rights of individual states to reject laws they deemed unconstitutional or against their interests. Nullification was particularly associated with the issue of slavery and the tension between the federal government and southern states.