Article I, Section 10, Clause 1:
No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.
At the time of the Civil War, the Court relied on the prohibition on treaties, alliances, or confederations in holding that the Confederation formed by the seceding states could not be recognized as having any legal existence. 1 Today, the prohibition's practical significance lies in the limitations that it implies upon the power of the states to deal with matters having a bearing upon international relations.
In the early case of Holmes v. Jennison, 2 Chief Justice Taney invoked it as a reason for holding that a state had no power to deliver up a fugitive from justice to a foreign state. More recently, the kindred idea that the responsibility for the conduct of foreign relations rests exclusively with the Federal Government prompted the Court to hold that, because the oil under the three-mile marginal belt along the California coast might well become the subject of international dispute, and because the ocean, including this three-mile belt, is of vital consequence to the nation in its desire to engage in commerce and to live in peace with the world, the Federal Government has paramount rights in and power over that belt, including full dominion over the resources of the soil under the water area. 3 In Skiriotes v. Florida, 4 the Court, on the other hand, ruled that this clause did not disable Florida from regulating the manner in which its own citizens may engage in sponge fishing outside its territorial waters. Speaking for a unanimous Court, Chief Justice Hughes declared,
When its action does not conflict with federal legislation, the sovereign authority of the State over the conduct of its citizens upon the high seas is analogous to the sovereign authority of the United States over its citizens in like circumstances. 5