Whether federal law on federal enclaves borrows current state law, rather than state law only as it existed when the enclave was created

The Federal Enclave Clause authorizes Congress to “exercise exclusive Legislation in all Cases whatsoever” over federal enclaves. U.S. Const. art. I, §8, cl.17. That grant of pl enary legislative authority, however, does not prevent Congress from borrowing state law to provide a rule of decision on federal enclaves. Instead, this Cour t has held for well over a century that when the federal government acquires a federal enclave, federal law on that enclave borrows state law that is not inconsistent with controlling federal law. In the decision below, the Fifth Circuit recognized that federal law typically bo rrows state law to provide a rule of decision in federal enclaves. But based on language in this Court’s prior decisions, it held—in agreement with other circuits, but contrary to at least four state supreme courts—that federal law borrows state law only as it existed when the federal government acquired the enclave , regardless of any subsequent changes in that state law. That approach not only conflicts with how federal law borrows state law in all other contexts, but would subject millions of military servicemembers and their families—to an unpredictable patchwork regime under which each federal enclave across the country borrows its own distinct (and distinctly outdated) version of state law. The question presented is: Whether federal law on federal enclaves borrows current state law, rather than state law only as it existed when the en clave was created.