Hager v. Thomson et al. was a case heard by the United States Supreme Court in 1861 that dealt with the issue of slavery and whether or not it could be enforced in territories acquired from Mexico after the Mexican-American War (1846–48). The plaintiff, Hager, argued that he had been illegally enslaved while living in one of these territories and sought his freedom through this suit. The defendants argued that since slavery had been recognized as legal under Spanish colonial rule prior to American acquisition of the territory, they were entitled to continue its practice there afterwards. In a 5-4 decision, Chief Justice Roger B Taney wrote for the majority opinion which held that Congress did not have authority over slavery within those newly acquired territories; instead it fell upon each state legislature to decide if they would allow or prohibit it within their borders. This ruling effectively overturned an earlier precedent set by Dred Scott v Sanford (1857) which declared slaves as non-citizens without any rights whatsoever regardless of where they lived or what laws governed them at any given time

In Hager v. Thomson et al., the Supreme Court was asked to decide whether a state court had jurisdiction over a case involving citizens of different states. The majority opinion held that the state court did have jurisdiction, but Justice Grier dissented from this decision. He argued that under Article III of the Constitution, federal courts were given exclusive original jurisdiction in cases where citizens of different states are involved and thus it is unconstitutional for a state court to exercise such authority. Furthermore, he noted that Congress has not granted any power or authority to allow state courts to hear these types of cases and therefore they should be heard by federal courts only. In conclusion, Justice Grier believed that allowing a state court to hear this type of case would violate both constitutional law as well as congressional intent and thus should not be allowed.