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The 'We Are Still In' movement raised novel and urgent questions about the status of executive agreements, treaties, and customary international law in U.S. courts. As sub-national governments increasingly face difficult trade-offs between climate change mitigation and adaptation, American courts will confront challenges thereto likely grounded in various types of "dormant" preemption of state and local initiatives. This symposium essay argues that our courts must first situate sub-national actions on climate mitigation within a complex and evolving context of mitigation as a globally-scaled collective good that can only be provided if contributions thereto accumulate over time. They must also avoid lumping sub-national action advantaging some firms in a competitive market under the old unionist fears of protectionism in all things local. A bottom-up movement like 'We Are Still In' may be uniquely positioned to keep the nation's pledges on climate change mitigation — and its obligations under U.S. treaties and public international law — but only if our courts understand the stakes and the fine print of our foreign affairs federalism.