Dred Scott v. Sandford, 60 U.S. (19 How.) 393 (1857), was a landmark decision of the United States Supreme Court that held the United States Constitution did not extend American citizenship to people of black African descent, and therefore they could not enjoy the rights and privileges the Constitution conferred upon American citizens. The decision is widely considered the worst in the Supreme Court's history, widely denounced for its overt racism, judicial activism, and poor legal reasoning. It de jure nationalized slavery, and thus played a crucial role in the events that led to the American Civil War four years later. Legal scholar Bernard Schwartz said that it "stands first in any list of the worst Supreme Court decisions." Chief Justice Charles Evans Hughes called it the Court's "greatest self-inflicted wound".

The decision involved the case of Dred Scott, an enslaved black man whose owners had taken him from Missouri, a slave-holding state, into Illinois and the Wisconsin Territory, where slavery was illegal. When his owners later brought him back to Missouri, Scott sued for his freedom and claimed that because he had been taken into "free" U.S. territory, he had automatically been freed and was legally no longer a slave. Scott sued first in Missouri state court, which ruled that he was still a slave under its law. He then sued in U.S. federal court, which ruled against him by deciding that it had to apply Missouri law to the case. He then appealed to the U.S. Supreme Court.

In March 1857, the Supreme Court issued a 7–2 decision against Scott. In an opinion written by Chief Justice Roger B. Taney, the Court ruled that people of African descent "are not included, and were not intended to be included, under the word 'citizens' in the Constitution, and can therefore claim none of the rights and privileges which that instrument provides for and secures to citizens of the United States"; more specifically, that African Americans were not entitled to "full liberty of speech ... to hold public meetings ... and to keep and carry arms" along with other constitutionally protected rights and privileges. Taney supported his ruling with an extended survey of American state and local laws from the time of the Constitution's drafting in 1787 that purported to show that a "perpetual and impassable barrier was intended to be erected between the white race and the one which they had reduced to slavery." Because the Court ruled that Scott was not an American citizen, he was also not a citizen of any state and, accordingly, could never establish the "diversity of citizenship" that Article III of the U.S. Constitution requires for a U.S. federal court to be able to exercise jurisdiction over a case. After ruling on those issues surrounding Scott, Taney struck down the Missouri Compromise because, by prohibiting slavery in U.S. territories north of the 36°30′ parallel, it interfered with slave owners' property rights under the Fifth Amendment to the United States Constitution.

Although Taney and several other justices hoped the decision would settle the slavery controversy, which was increasingly dividing the American public, the decision only exacerbated interstate tension. Taney's majority opinion suited the slaveholding states, but was intensely decried in all the other states. The decision inflamed the national debate over slavery and deepened the divide that led ultimately to the American Civil War. In 1865, after the Union's victory, the Court's ruling in Dred Scott was superseded by the passage of the Thirteenth Amendment to the U.S. Constitution, which abolished slavery, and the Fourteenth Amendment, whose first section guaranteed citizenship for "[a]ll persons born or naturalized in the United States and subject to the jurisdiction thereof."

Historians agree that the Court decision was a major disaster for the nation as it dramatically inflamed tensions leading to the Civil War. The ruling is widely considered to have had the intent of bringing finality to the territorial crisis resulting from the Louisiana Purchase by creating a constitutional right to own slaves anywhere in the country, while permanently disenfranchising all people of African descent, in an act generally believed to constitute blatant judicial activism. Often cited in proof of this is the court's decision to go out of its way to overturn the Missouri Compromise, which had already been replaced with the Kansas–Nebraska Act and thus was a legally moot issue, because the latter act was determined by the due process of popular sovereignty, and thus could not be overturned the same way as the Missouri Compromise. During the United States election of 1860, Republicans rejected the ruling as being corrupted by partisanship and non-binding because the court had no jurisdiction. Their presidential nominee, Abraham Lincoln, stated he would not permit slavery anywhere in the country except where it already existed, which directly contradicted the court's ruling. His election is considered the final event that led the Southern states to secede from the Union, igniting the American Civil War.

In the late 1810s, a major political dispute arose over the creation of new U.S. states from the vast territory the United States had acquired from France in 1803 by the Louisiana Purchase. The dispute centered on whether the new states would be "free" states in which slavery would be illegal, as in the Northern states, or whether they would be "slave" states in which slavery would be legal, as in the Southern states.

In 1820, the U.S. Congress passed legislation known as the "Missouri Compromise" that was intended to resolve the dispute. The Compromise first admitted Maine into the Union as a free state, then created Missouri out of a portion of the Louisiana Purchase territory and admitted it as a slave state; it also prohibited slavery in the area north of the parallel 36°30′ north, where most of the territory lay. The legal effects of a slaveowner taking his slaves from Missouri into the free territory north of latitude 36°30′ north, as well as the constitutionality of the Missouri Compromise itself, eventually came to a head in the Dred Scott case.^{[citation needed]}

Dred Scott was born a slave in Virginia around 1799. Little is known of his early years. His owner, Peter Blow, moved to Alabama in 1818, taking his six slaves along to work a farm near Huntsville. In 1830, Blow gave up farming and settled in St. Louis, Missouri, where he sold Scott to U.S. Army surgeon Dr. John Emerson. After purchasing Scott, Emerson took him to Fort Armstrong in Illinois. A free state, Illinois had been free as a territory under the Northwest Ordinance of 1787 and had prohibited slavery in its constitution in 1819 when it was admitted as a state.