De Liano v. Gaines was a United States Supreme Court case that dealt with the issue of whether a state court had the authority to issue a writ of mandamus to compel a state officer to perform a duty. The case arose when the plaintiff, De Liano, sought to compel the defendant, Gaines, to issue a license to practice law in the state of California. The Supreme Court held that the state court did not have the authority to issue a writ of mandamus to compel a state officer to perform a duty. The Court reasoned that the writ of mandamus was a prerogative of the federal government, and that the state court did not have the power to issue such a writ. The Court further held that the state court could not interfere with the exercise of the federal government's power to issue writs of mandamus. The Court's decision in De Liano v. Gaines established that state courts do not have the authority to issue writs of mandamus to compel state officers to perform their duties. This decision has been cited in numerous subsequent cases, and has been used to support the principle that state courts cannot interfere with the exercise of the federal government's power to issue writs of mandamus.

In De Liano v. Gaines, the United States Supreme Court was tasked with determining whether a state court had jurisdiction over an action brought by a non-resident against another non-resident in regards to land located within the state. The majority opinion held that such jurisdiction did exist and affirmed the decision of the lower court. However, Justice Field dissented from this opinion on two grounds: firstly, he argued that there was no precedent for allowing states to exercise jurisdiction over actions between two non-residents; secondly, he maintained that it would be unconstitutional for states to do so as it would violate Article IV Section 1 of the Constitution which guarantees citizens “full faith and credit” in other states’ judicial proceedings. He concluded his dissent by stating that if Congress wished to grant such power then they should pass legislation explicitly granting it rather than relying on courts interpreting existing laws broadly enough to allow them this authority.