The historical processes through which international law became, conceptually, a universal legal order and, geographically, an order with a global scope of validity, are long and complex. These transformations, which began to appear during the second half of the nineteenth century, did not end until post-War-World-II decolonization. This article examines one particular aspect of these transformations: once non-Western states were admitted and begun to participate in the international community, did the rules of international law governing the interaction between Western and non-Western States change? What did it mean for semi-peripheral states to acquire sovereignty? The article argues that during the first decades of the twentieth century, semi-peripheral lawyers realized that sovereignty, so longed-for during the nineteenth century, conferred, under classical international law, much less autonomy and equality than they had anticipated. Moreover, at the turn of the century, the specific challenges faced by semi-peripheral states in their interaction with Western powers shifted, so that classical international law exhausted its power and stopped being useful. The article thus offers, from the perspective of the semi-periphery, an explanation of the shift from classical to modern international law.