This essay responds to the Supreme Court’s recent decision in Arizona v. United States, which struck down all but one of the disputed sections of Arizona’s S.B. 1070 immigration law. It advances the theory that although the Arizona Court purported to apply classic conflict and field preemption analyses, it was actually using a different form of preemption, one that gives particular weight to federal interests where questions of national sovereignty are at stake. The Court did so through doctrinal borrowing of the “plenary power doctrine,” which gives the political branches special deference when passing or executing immigration legislation, even where doing so would otherwise violate individual constitutional rights. This Essay labels the form of preemption used in Arizona and other alienage cases “plenary power preemption.” It shows how this doctrine developed over time, as the scope of the legitimate exercise of state police power and federal immigration changed, and federal and state regulation of noncitizens became more complex and enmeshed. It argues that plenary power preemption has two important effects: it allows courts to evade the thorny question of the scope of executive—as opposed to legislative—power over immigration, and it substitutes for the lack of an equal protection doctrine that adequately protects unauthorized aliens from discrimination.