Contemporary equal protection doctrine touts the principle of congruence: the notion that equal protection means the same thing whether applied to state or to federal laws. The federalism-tinged equal protection analysis at the heart of Justice Kennedy’s opinion in United States v. Windsor, however, necessarily violates the congruence principle. Commentators and courts—especially those deciding how Windsor’s federalism should affect the ever-growing number of state same-sex marriage cases—have so far failed to account for Windsor’s noncongruent equal protection, much less ask whether noncongruence is generally desirable, and if so, what form it should take.

This Article draws answers to those questions from the Supreme Court’s alienage discrimination cases, which offer three distinct models of noncongruence, each of which is reflected in Windsor. The alienage cases show that instead of applying different levels of scrutiny to federal and state laws, a better understanding of noncongruence would allow different levels of government to assert different interests in defending their laws. By reconstructing and evaluating the ways that structure and rights intersect in the alienage cases, this Article considers for the first time what the return of noncongruent equal protection could mean both for cases that follow Windsor and for equal protection doctrine more broadly.