This chapter aims to locate the critical juncture when securitization is – from a moral point of view – not merely permitted (and thus optional) but obligatory (mandatory). This chapter argues that securitization is morally obligatory (pro tanto) when ‘must cause’ is satisfied. This is the case when would-be securitizing actors have tried relevant less harmful alternatives and when these have failed to satisfy just cause. This view is in line with supererogationism a philosophical position that sees value in keeping morality – where possible – free from prescriptive behaviour whereby every good and right generates a moral obligation. It is argued that the value of autonomy that allows relevant actors (limited) freedom to choose on how to respond to a just cause for securitization diminishes as certainty that securitization is the best response increases. Certainty increases subject to evidence that less harmful options than securitization do not work. This chapter goes on to discuss must cause in detail on six illustrative hypothetical examples of different threat types, including drought disaster, cyberattack, and climate disaster.