Dred Scott was important primarily because of his court case, Dred Scott v. Sandford (1857), which became a landmark decision by the United States Supreme Court. In this case, Scott, an enslaved African American man, argued that he should be free because he had lived in free territories. However, the Supreme Court ruled against him, stating that African Americans could not be considered citizens and therefore had no right to sue in federal court. The Court's decision also declared that the federal government had no authority to regulate slavery in the territories, exacerbating sectional tensions and contributing to the onset of the Civil War.
Therefore, the most accurate statement regarding Dred Scott's importance is: He was a slave who argued he was free.