In re Neagle, Petitioner is a Supreme Court case from 1890 that dealt with the issue of federal protection of a state witness. The petitioner, David Neagle, was a U.S. Marshal who had been hired by the state of California to protect a witness in a criminal trial. Neagle shot and killed the witness’s assailant, and was subsequently charged with murder by the state of California. Neagle argued that he was acting in his capacity as a federal officer and was therefore immune from prosecution. The Supreme Court ultimately ruled in favor of Neagle, holding that the federal government had the right to protect its officers from state prosecution when they were acting in their official capacity. The Court reasoned that the federal government had a duty to protect its officers from state interference, and that the state of California had no authority to prosecute Neagle for his actions. The Court also held that the federal government had the right to protect its officers from state interference, even when the officer was acting in a state capacity. The ruling in In re Neagle, Petitioner established the principle of federal protection of state witnesses, and has been cited in numerous subsequent cases. The ruling has been used to protect federal officers from state prosecution when they are acting in their official capacity, and has been used to protect state witnesses from federal interference.

In In re Neagle, Petitioner, the Supreme Court issued a dissenting opinion that argued against the majority's decision to grant immunity from prosecution for Deputy United States Marshal David Neagle. The dissenters believed that granting such immunity would set a dangerous precedent and undermine public confidence in law enforcement officers. They argued that it was not appropriate for the court to interfere with an ongoing criminal investigation or trial by granting immunity without any clear legal authority or justification. Furthermore, they noted that there were other ways of protecting federal officers from liability while still allowing them to perform their duties without fear of retribution. Ultimately, they concluded that this case should have been left up to Congress and state legislatures rather than being decided by judicial fiat.