This chapter argues that the systemic integration of customary international law in the interpretation of investment treaties should equip vulnerable polities with principled arguments for why a high degree of deference is warranted in their adoption of regulatory measures to address the health, economic and social dimensions of chronic crises. The chapter reviews the recent demand for governments to protect their right to regulate by imposing a moratorium on investment treaty claims. This critical discourse has overlooked the deeper roots of the State’s presumptive right to regulate without compensation, an expression of its territorial sovereignty under customary international law. Evident across landmark decisions such as the Oscar Chinn Case, the Iron Rhine Arbitration and Philip Morris v. Uruguay, the regulatory dimension of territorial sovereignty both preceded and endured the mid-century debate over the standard of compensation for nationalisation and the subsequent rise of investment arbitration. In contemporary practice, the customary rule that there is no State responsibility for reasonable regulation may be integrated through arbitral practice to ensure each claimant bears the burden to prove that regulatory measures were unreasonable, discriminatory, or adopted in bad faith.