At the time of the Civil War, the answer to the question was uncertain. During the War of 1812, for example, some areas in the north-east explored secession at the 1814 Hartford Convention, and through to the 1850s, there was a doctrine of 'nullification' in which states asserted they could 'nullify' federal laws they thought were unconstitutional on the basis that the union was a voluntary compact between states (which may have served to temper secessionist impulses). Nullification was struck down by the US Supreme Court in 1859 in Ableman v Booth.

The question of whether secession was legal was only resolved legally in 1869 by the US Supreme Court definitively in Texas v White, in which it was held that secession was unlawful.