Written by attorneys · grounded in primary & secondary sources — see below
A stipulation between two or more states or between a state and a foreign power. Such an arrangement requires the consent of Congress when it tends to increase the political power of the participating states in a manner that may encroach upon or interfere with the just supremacy of the United States.
Virginia and Tennessee appoint commissioners to survey and mark their shared boundary without first obtaining congressional approval. The states later seek judicial enforcement of the resulting line. The court must decide whether the arrangement constitutes an agreement or compact that needed prior congressional consent.
Oregon conditions an alien's right to inherit property on the existence of reciprocal inheritance rights in the alien's home country and on the absence of confiscatory policies there. The statute produces incidental effects on foreign relations. The Court examines whether these effects amount to an agreement or compact with foreign powers that requires congressional consent.
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A bank chartered in Georgia seeks to conduct business in Alabama without obtaining a local charter. Alabama officials challenge the bank's authority. The Court considers whether the bank's operations rest on an implicit agreement or compact between the two states that would require congressional approval.
Treaties, alliances, and confederations are absolutely forbidden to the states. Agreements and compacts may be made with congressional consent. The distinction turns on whether the arrangement is political in nature and tends to increase state power at the expense of federal supremacy.
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Consent is required when the agreement is directed to the formation of any combination that tends to increase the political power of the states and may encroach upon or interfere with federal supremacy. Not every agreement between states triggers the requirement.
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No. Only those that encroach on federal supremacy require congressional consent. The national government alone controls foreign relations, so states lack authority to adjust controversies with foreign powers through agreements that affect national interests.
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…S., at 517. The Court noted that California had not violated any express command of the Constitution by entering into a treaty, agreement or compact with foreign countries. It said that "[w]hat California has done will have some incidental or indirect effect in foreign countries. But that is true of many state laws which none would…