In the United States Supreme Court case of The United States v. John C. Fremont, the court was tasked with determining whether or not a military officer had exceeded his authority in issuing an emancipation proclamation to free slaves in California during the Mexican-American War. At issue was whether Fremont's actions were within his powers as a military commander and if they violated any existing laws at that time. Ultimately, the court found that while Fremont did have some discretion when it came to enforcing martial law, he had gone beyond what Congress authorized him to do by freeing all slaves without their owners' consent and thus acted outside of his power as a military commander. As such, they ruled against him and declared his proclamation null and void under federal law.

In the United States v. John C. Fremont, the Supreme Court was asked to decide whether a California district court had jurisdiction over an action brought by the United States against Fremont for trespass and damages on public lands in California. The majority opinion held that it did not have such jurisdiction because Congress had not provided for any judicial proceedings in such cases at that time. However, Justice McLean dissented from this decision and argued that since Congress had granted exclusive authority to regulate public land matters to California courts, they should be allowed to hear this case as well. He further noted that if Congress intended otherwise, then it would have been easy enough for them to provide specific instructions regarding how these types of disputes were supposed to be handled instead of leaving it up entirely up to state courts or other federal authorities without providing any guidance whatsoever. In conclusion, he believed there was no legal basis upon which one could deny the district court's jurisdiction over this matter and thus voted against affirming the lower court's ruling on appeal