Loftin Cotton v. United States was a case heard by the US Supreme Court in 1850. The plaintiff, Loftin Cotton, had been convicted of piracy and sentenced to death for his involvement in an attack on a Spanish vessel off the coast of Cuba. He argued that he should not have been tried under US law since he was not an American citizen and did not commit any acts against US vessels or citizens; therefore, he claimed that his conviction violated the Constitution's provision prohibiting ex post facto laws from being applied retroactively. The court disagreed with him and held that Congress had authority to punish offenses committed outside its jurisdiction if they were considered crimes according to international law or treaties between nations. Furthermore, it found that there was no violation of constitutional rights because Cotton could have reasonably foreseen such punishment when committing the act in question. Ultimately, this ruling affirmed Congress' power over foreign affairs as well as their ability to enforce international agreements through domestic legislation

In Loftin Cotton v. United States, the Supreme Court was asked to decide whether a state court had jurisdiction over a case involving an alleged violation of federal law. The majority opinion held that it did not, as the Constitution does not grant states such authority and Congress has never given them permission to do so. However, Justice McLean dissented from this decision on two grounds: firstly, he argued that since Congress had passed laws allowing for suits in state courts against those who violated federal statutes or treaties, then they must have intended for states to have some degree of jurisdiction; secondly, he argued that if there were no power granted by either the Constitution or by Congress for states to exercise jurisdiction in these cases then it would be unconstitutional and therefore void. He concluded his dissent with a warning about how this ruling could lead to dangerous consequences if left unchecked - namely that individuals accused of violating federal law would be unable to receive justice through their local courts due its lack of authority over such matters.