Gunn v. Barry was a United States Supreme Court case that dealt with the issue of whether a state could impose a tax on a federal officer. The case arose when the state of Mississippi imposed a tax on the salary of a federal officer, William Gunn, who was appointed by the United States government to serve as a collector of customs in the state. Gunn refused to pay the tax, arguing that it was unconstitutional for a state to tax a federal officer. The Supreme Court agreed with Gunn, ruling that the state of Mississippi had no authority to impose a tax on a federal officer. The Court held that the power to tax federal officers was exclusively within the purview of the federal government, and that the state of Mississippi had no authority to interfere with the federal government's power to tax its officers. The Court's decision in Gunn v. Barry established the principle that states cannot interfere with the federal government's power to tax its officers.

In the case of Gunn v. Barry, Justice Field delivered a dissenting opinion in which he argued that the majority's decision was contrary to established precedent and would lead to an undesirable result. He noted that prior cases had held that when a contract is made between two parties, it must be performed according to its terms unless there are circumstances beyond either party's control preventing performance. In this case, he argued, no such circumstances existed; therefore, the defendant should have been required to perform his obligations under the contract as agreed upon by both parties. Furthermore, Justice Field stated that allowing one party to unilaterally terminate or modify a validly formed agreement without any legal justification would create uncertainty in contractual relationships and could potentially encourage fraud and other bad behavior on behalf of contracting parties who may seek ways out of their agreements whenever they become inconvenient or unprofitable for them.