Canadian Yearbook of International Law/Annuaire Canadien de droit international
The text of the US Constitution appears to require that individual states, to the extent that they are ever allowed to conclude agreements with foreign governments, must obtain congressional approval. In practice, however, states conclude many agreements with foreign governments, including with Canada and its provinces, and they almost never seek congressional approval. This practice is an illustration of both the importance of federalism in US foreign relations and the significant role played by historical practice in informing US constitutional interpretation. The phenomenon of state international agreements assumed new prominence in 2019 when the Trump administration sued to challenge a climate change agreement that the state of California had made with Québec. Despite this challenge, for the most part, neither Congress nor the executive branch has resisted the growth in state international agreements. This acquiescence could change as countries like China target US states in an effort to work around strained relations with the US national government and as states become more assertive in resisting the national government’s foreign policies. In any event, the practice of state international agreements unapproved by Congress rests in part on a distinction between binding and non-binding agreements that deserves greater scrutiny under both domestic and international law.
Curtis A. Bradley, "State International Agreements: The United States,Canada, and Constitutional Evolution," Canadian Yearbook of international Law / Annuaire Canadien de droit international (2023).