The treatment of foreign investors has remained one of the most debated issues in international investment law. At the centre of this debate are two competing approaches. The first, associated with Elihu Root and Cordell Hull, argues that foreign investors are entitled to an international minimum standard of protection regardless of the standards available under the host State's domestic law. The second, advanced by Carlos Calvo, maintains that foreign investors should enjoy no greater rights than domestic investors and should ordinarily rely on the legal system of the host State to resolve disputes. This paper examines the competing approaches and argues that the Calvo Doctrine remains a relevant framework for balancing foreign investment protection with State sovereignty. Using a doctrinal approach, it analyses the historical development, legal principles, case law, and academic debates surrounding both doctrines. It concludes that, despite its limitations, the Calvo Doctrine's emphasis on sovereignty and domestic legal processes continues to influence contemporary debates on investor protection and international investment law.