Morgan Byrne was a slave in Missouri who sued the state for his freedom. He argued that since he had been born into slavery, but then taken to Illinois and held there as a free man for several years before being brought back to Missouri, he should be considered a free man under the terms of the Northwest Ordinance of 1787. The Supreme Court disagreed with him and ruled against him on the basis that even though he had lived as a free man in Illinois, when he returned to Missouri it was still within its jurisdiction and therefore subject to its laws regarding slavery. This decision set an important precedent which established that slaves were not protected by federal law if they traveled between states or territories where different rules applied.

In Morgan Byrne v. The State of Missouri, the Supreme Court was asked to decide whether a state law that allowed for the execution of an individual who had been convicted and sentenced by a court in another state was constitutional. Justice McLean wrote a dissenting opinion arguing that such laws violated the Constitution's Full Faith and Credit Clause which requires states to recognize judicial proceedings from other states. He argued that if this law were upheld it would lead to chaos as each state could pass its own laws regarding criminal convictions from other states, leading to confusion and conflicting rulings among different jurisdictions. Furthermore, he argued that allowing one state’s courts jurisdiction over individuals in another would be unfair since those individuals have no representation or recourse within their home state’s legal system. Ultimately, Justice McLean concluded that such laws should not be permitted under the Constitution because they violate both due process rights as well as principles of comity between sovereign nations (states).