In 2006, the Department of Homeland Security (DHS) asserted federal preemption of state law governing the security of chemical facilities. The continuing controversy over chemical security preemption reveals one way in which executive power asserts itself in the national security context: the reclassification of seemingly domestic regulatory concerns as matters of national security and the consequent constriction of state regulatory authority. This Note analyzes the DHS’s chemical security regulations as a case study for the problem of national security preemption. It argues that the presumption of federal supremacy in foreign affairs can ratify conclusory and unsupported preemption claims because the national security interest mixes both foreign and domestic affairs, while the only doctrinal guidance for defining that interest comes from contested foreign affairs preemption doctrines. The Note proposes that, if strengthened, deference doctrines drawn from administrative law provide the best means of scrutinizing and limiting such claims of executive authority. Agency claims of preemption on the basis of national security should be subject to heightened scrutiny. Such scrutiny is more useful than the stalemated positions of the law and security debate for policing the state-federal divide in national security.