Roman property law and Roman contract law as well as the property centered Roman ethics put forth by Cicero in several of his works were the traditions Grotius drew upon in developing his natural rights system. While both the medieval just war tradition and Grotius's immediate political context deserve scholarly attention and constitute important influences on Grotius's natural law tenets, it is a Roman tradition of subjective legal remedies and of just war which lays claim to a foundational role with regard to his conception of subjective natural rights. Grotius made use of Roman law and Roman ethics in order to submit a normative case for a rights-based just war in the East Indies. His conception of a law of nature was originally conceived to apply a theory of compensatory justice to the high seas of Southeast Asia, envisaged as a natural state lacking political authority. Eventually, however, this argument was to reveal its anti-absolutist implications, and contributed—by virtue of its applicability to individuals, private entities and commonwealths alike—to the emergence of a rights-based constitutionalism. This article discusses Grotius's early treatise De iure praedae commentarius (1604-1606) and its offshoot Mare liberum, which already contained an inchoate version of subjective natural rights, as well as the elaborate natural rights doctrine which can be found in Grotius's early Theses LVI and in the Defensio capitis quinti maris liberi, a defense of the fifth chapter of Mare liberum, written around 1615 and directed against the Scottish jurist William Welwod's attack on Mare liberum.