The Protector was a Supreme Court case from 1869 that dealt with the issue of whether a foreign-built vessel could be registered as a United States vessel. The case was brought by the owners of the Protector, a British-built vessel, who sought to register it as a United States vessel. The Supreme Court held that the vessel could not be registered as a United States vessel because it had been built in a foreign country. The Court reasoned that the vessel was not built in the United States and therefore could not be registered as a United States vessel. The Court also held that the vessel was not entitled to the protection of the United States flag because it had not been built in the United States. The Court reasoned that the vessel was not entitled to the protection of the United States flag because it had not been built in the United States and therefore could not be registered as a United States vessel. The Court also held that the vessel was not entitled to the protection of the United States flag because it had not been built in the United States. The Court reasoned that the vessel was not entitled to the protection of the United States flag because it had not been built in the United States and therefore could not be registered as a United States vessel. The Court's decision in The Protector established that a foreign-built vessel could not be registered as a United States vessel and was not entitled to the protection of the United States flag. This decision has been cited in numerous subsequent cases and has been used to support the principle that a foreign-built vessel cannot be registered as a United States vessel.

In The Protector, the Supreme Court was asked to decide whether a vessel called "The Protector" could be seized and sold in order to satisfy a judgment against its owner. The majority opinion held that it could not because the vessel had been registered as an American ship under federal law. However, Justice Field dissented from this decision on two grounds: first, he argued that the registration of vessels did not confer any special privileges or immunities upon them; second, he maintained that Congress had no authority to pass laws regulating maritime commerce between states. He concluded by stating his belief that such matters should be left up to state legislatures instead of being regulated by federal legislation.