Mahan v. United States was a Supreme Court case in which the Court held that the United States had the right to take possession of land in the Indian Territory without compensation. The case arose when the United States government sought to take possession of land in the Indian Territory for the purpose of constructing a railroad. The land was owned by the Creek Nation, and the Nation refused to give up the land without compensation. The United States argued that it had the right to take possession of the land without compensation under the Treaty of 1866, which had been signed between the United States and the Creek Nation. The Supreme Court held that the United States had the right to take possession of the land without compensation. The Court reasoned that the Treaty of 1866 gave the United States the right to take possession of the land for the purpose of constructing a railroad, and that the United States was not obligated to pay compensation for the land. The Court also held that the United States had the right to take possession of the land without the consent of the Creek Nation, as the Treaty of 1866 did not require the consent of the Creek Nation for the taking of the land. In conclusion, the Supreme Court held that the United States had the right to take possession of land in the Indian Territory without compensation under the Treaty of 1866. The Court reasoned that the Treaty of 1866 gave the United States the right to take possession of the land for the purpose of constructing a railroad, and that the United States was not obligated to pay compensation for the land. The Court also held that the United States had the right to take possession of the land without the consent of the Creek Nation.

In the case of Mahan v. United States, Justice Field delivered a dissenting opinion in which he argued that Congress had exceeded its authority under the Constitution by passing legislation to punish an individual for acts committed prior to the passage of said law. He argued that such retroactive laws were unconstitutional and violated due process rights as they deprived individuals of their liberty without notice or opportunity to be heard on their behalf. Furthermore, he noted that this type of ex post facto law was not only prohibited by the Constitution but also contrary to fundamental principles of justice and fairness. In conclusion, Justice Field concluded that Congress did not have authority over matters concerning criminal procedure and punishment unless specifically granted in Article I Section 8 Clause 18 (the Necessary & Proper Clause).