Final answer:
Roger B. Taney gave two main reasons for the Supreme Court's decision in the Dred Scott case: African Americans could not be U.S. citizens, and residence in a free area did not confer freedom upon returning to slave territory; also, Congress lacked the authority to prohibit slavery in the territories.
Step-by-step explanation:
Reasons Behind the Dred Scott Decision
According to Roger B. Taney, there were two main reasons that the court decided to rule that Dred Scott was not a free man. First, Scott had no standing to sue in federal court because, according to Taney, blacks could not be citizens of the United States. This decision was rooted in Taney's interpretation of the Constitution and the intent of its framers, implying that the framers never intended to include African Americans under the protections of the Constitution. Second, Taney declared that residence in a 'free territory' did not confer freedom on an enslaved person who returned to 'slave territory,' reversing the presumption that living in a free area could change an enslaved person's legal status.
Furthermore, the Supreme Court ruling stated that Congress had no authority to bar slavery in the territories, rendering the Missouri Compromise unconstitutional. This comprehensive approach attempted to address the larger issues of slavery and its extension into American territories, significantly impacting the national debate on slavery.