In the 1909 case Watson v. State of Maryland, the U.S. Supreme Court upheld a conviction for practicing medicine without a license in Maryland against Dr. John H. Watson, an African American doctor who had been licensed to practice in Pennsylvania and Washington D.C., but not in Maryland where he was treating patients at his home office during weekends while maintaining his weekday practice in D.C.. The court ruled that states have the power to regulate medical practices within their borders as part of their police powers and can require doctors from other states to obtain licenses before they are allowed to practice medicine there. This decision affirmed state's rights over professional licensing regulations and reinforced public health protections by ensuring practitioners meet local standards.

In the dissenting opinion for Watson v. State of Maryland, Justice Harlan argued that the majority's decision was a dangerous precedent that undermined individual liberties and constitutional rights. He contended that by upholding Maryland's law requiring physicians to register with the state before practicing medicine, the court had effectively given states unlimited power to regulate professions - a power he believed they did not possess under the Constitution. Furthermore, he expressed concern about potential abuses of this power by states in future cases and warned against allowing such broad regulatory authority without clear constitutional justification or safeguards against misuse. In his view, while it is important for states to protect public health and safety through reasonable regulation of medical practice, these goals must be balanced against individuals' right to pursue their chosen profession free from undue governmental interference.