The Treaty of Lausanne, signed on 24 July 1923, provided the international legal framework that consolidated the emergence of the Republic of Turkey as the sovereign successor to the Ottoman Empire following war, occupation, and nationalist revolution. Rather than creating a new political order, the Treaty confirmed and stabilised outcomes produced by military victory and imperial collapse. It delineated Turkey’s borders, abolished the Capitulations, resolved questions of sovereignty and debt, regulated the Straits, and established an internationally supervised regime for the protection of non-Muslim minorities. Alongside, but legally distinct from, the Treaty, the Convention Concerning the Exchange of Greek and Turkish Populations institutionalised compulsory demographic transfer as a mechanism of state-building. This chapter argues that Lausanne’s legacy is fundamentally ambivalent. While it marked a decisive moment in post-imperial state formation and affirmed principles of territorial integrity and sovereign equality, it simultaneously exposed the limits of minority protection and international guarantees. The Treaty’s endorsement of demographic engineering, the asymmetrical nature of minority obligations, and the contingent survival of institutions such as the Ecumenical Patriarchate reveal the tensions inherent in constructing a new international order grounded in the nation-state. Lausanne thus stands as both a cornerstone of modern international law and a cautionary illustration of its structural constraints.