What does indivisible mean in the Pledge?

In the Pledge of Allegiance, the word indivisible means that the nation cannot be lawfully divided or split apart. The phrase one Nation under God, indivisible declares that the United States is a single political entity made up of fifty states under one constitutional government, not a loose confederation that states may quit at will.

Indivisibility was the central question of the Civil War. Beginning with South Carolina's secession on December 20, 1860 and continuing through the formation of the Confederate States of America in February 1861, eleven southern states attempted to leave the Union over slavery, states' rights, and political control. President Abraham Lincoln rejected the legitimacy of secession in his First Inaugural Address on March 4, 1861, declaring that the Union of these States is perpetual and that no State, upon its own mere motion, can lawfully get out of the Union.

The Civil War, which lasted from 1861 to 1865 and cost approximately 750,000 lives, settled the question by force. The Supreme Court confirmed the result in Texas v. White (1869), where Chief Justice Salmon Chase wrote that the Constitution, in all its provisions, looks to an indestructible Union, composed of indestructible States.

Francis Bellamy chose the word indivisible deliberately when he wrote the original Pledge in 1892, only twenty-seven years after Appomattox and within living memory of the war. Veterans of both armies still walked the streets, and many of the children reciting Bellamy's Pledge had fathers who had fought. Pairing the word with one Nation gave the Pledge a Civil War-era understanding of American unity, an answer to the great constitutional crisis of the nineteenth century. The phrase under God was added by Congress in 1954, but the original indivisible has stood unchanged since 1892.

The principle of indivisibility shapes American constitutional law in subtle ways. The Constitution does not provide a procedure for a state to secede, the Tenth Amendment reserves powers to the states only when not denied by the Constitution, and the Supremacy Clause of Article VI makes federal law the supreme Law of the Land. States retain wide authority over their own affairs, but they belong to the Union permanently. Naturalization candidates who recite the Pledge are affirming this constitutional reality of perpetual union.