This article discusses some of the challenges related to the beginning of application of international humanitarian law (IHL). It concludes that IHL pertaining to international armed conflicts begins to apply as soon as one State employs force in the territory of another State without the latter's consent, provided that the violence is of a collective nature. In the case of non-international armed conflicts, this article acknowledges that it is now well settled that the two key criteria are the organization of the parties to the conflict and the level of intensity of the violence. This article shows however that some of the challenges inherent to the beginning of application of IHL make it almost impossible to identify a very single point in time at which it begins to become applicable, be it for international