The concept of due diligence is related to the theory of international obligations. The simple but yet complex idea is that diligence is an element of certain primary standards of the State, including the obligations of prevention. Its scope is limited to situations where the State is required to prevent or suppress certain damage. Rooted in the Roman system of law through the figure of the bonus pater familias, due diligence appears in the international legal order first in the area of neutrality before experiencing a fortune in other areas including the protection of foreigners, the security of foreign States, human rights, the environment. This article aims to demonstrate that due diligence has grown from a simple rule of neutrality to a customary norm of general international law before acquiring today the status of general principle applicable even in the absence of specific injunction of a primary standard. Then, it revisits the famous Alabama case to show that the legal regime of neutrality which has fully emerged in the mid eighteenth century was also the point of effervescence of the concept of due diligence in the international legal order.