Using California’s self-consciously internationalist approach to climate change regulation as a primary example, this Article examines constitutional limitations on state foreign affairs activities. In particular, by focusing on the prospect of California’s establishment of a greenhouse gas (GHG) emissions trading system and its eventual linkage with comparable systems in Europe and elsewhere, this Article demonstrates that certain constitutional objections to extrajurisdictional linkage of state GHG emissions trading systems and the response that these objections necessitate may be more complicated than previously appreciated. First, in order to successfully combat the argument that state-level climate change activities interfere with a federal executive position of withholding binding domestic GHG emissions reductions in advance of a multilateral agreement including key developing nations, states must demonstrate that the executive branch is not acting with congressional support and has, furthermore, declared its position too informally to constitute an exercise of any of the president’s independent constitutional powers. Second, state efforts to link GHG emissions trading systems with those of other nations may face serious challenges under the foreign affairs and Foreign Commerce Clause doctrines. Finally, states’ efforts to integrate with other trading schemes or to otherwise protect the integrity of their own trading schemes must be carefully constructed lest they invite challenge as being discriminatory or overreaching, in light of more conventional dormant Commerce Clause constraints on state regulation.