Alexander Levi, a free black man from Pennsylvania, brought suit against John Thompson and other defendants in the Supreme Court of the United States. The case arose when Levi was arrested by Thompson for alleged violations of Maryland's slave laws. At issue was whether or not states had authority to arrest and detain individuals who were legally free under their own state's laws but were accused of violating another state's slave laws. In an opinion written by Justice Joseph Story, the court held that states did not have such authority because it would be unconstitutional for one state to interfere with another state’s rights over its citizens. Furthermore, since slavery is illegal in all U.S territories according to Article IV Section 2 Clause 3 of the Constitution (the “Fugitive Slave Clause”), any attempt at enforcing a foreign law regarding slavery within those territories would also be unconstitutional as well as contrary to public policy. As such, Alexander Levi won his case and was released from custody without having been charged with any crime whatsoever

In the case of Alexander Levi v. John Thompson et al., Chief Justice Taney delivered a dissenting opinion that argued in favor of the plaintiff, Alexander Levi. He asserted that although it was true that Maryland had passed an act to emancipate slaves within its borders, this did not necessarily mean they were free everywhere else. The court should have taken into consideration whether or not there was any evidence to suggest that Mr. Levi had been legally emancipated outside of Maryland and thus entitled to his freedom elsewhere as well. Furthermore, he argued that even if Mr. Levi's emancipation papers from Maryland were found invalid by other states, he still deserved protection under the Constitution since slavery is prohibited in all states regardless of their laws on emancipation and manumission (the voluntary freeing of a slave). In conclusion, Chief Justice Taney believed it was wrong for the court to deny Mr. Levi his right to freedom without taking into account all relevant facts surrounding his situation and urged them instead to consider whether or not he could be considered legally freed according to both state law and federal constitutional protections against slavery before making their final decision on this matter