Deliberate attacks on websites and computer networks, so-called cyber attacks, are part of life on the Internet, with some foreign governments encouraging, assisting, or directing such attacks. In a serious international crisis, similar attacks might be launched with dire consequences. Many prominent commentators insist that retaliating in cyberspace is constrained by the norms of the law of armed conflict, which were designed to protect civilians in conventional conflicts. If we repudiate some of these inappropriate constraints, we can deter cyber attacks by threatening to retaliate in kind. Older understandings of international law—notably those familiar to the American founders—provide better guidance for coping with the challenges of contemporary cyber conflict.