This article investigates the formal purpose of declaring wars for Hugo Grotius. Grotius was adamant that states always use justification in a duplicitous way to conceal their real motivation to go to war. As such, the purpose of declaration is not to assert the just cause of war. Rather, what any public declaration does, is provide recognition that confers legal validation to the disputing parties. The legal rules of war were described by the law of nations and occasionally permitted states to commit certain ‘war crimes’ with impunity. For Grotius, this was not a moral sanctioning of such crimes but rather a means to prevent the occurrence of wars, which such endless repudiations risked causing or exacerbating. Grotius’s concern for the effects of war is conspicuous; and recounting his maxim that war should always be a last resort, this article argues that declaration of war has a distinct moral purpose for Grotius. In fact, public declaration of war is, together with constraints on the conduct of war, a ‘principle of moderation’ Grotius insists should be upheld in times of war. Declaration of war gives the parties avenues to seek peace and reconciliation, and, therefore has a humanitarian purpose for Grotius by ultimately seeking to prevent the disparaging effects of war. Even in wars that do not demand a public declaration, such as those regulated by natural law such as punitive wars or defensive wars, Grotius cautions that declarations of war are advisable. The remaining lingering issue is how to engage with unlawful enemy combatants, such as pirates - a distinct problem that the international community still faces with increasing regularity. Grotius was certainly aware of the legal (and moral) consequences of recognising belligerency we cannot possible hope to build moral relations with, and, this article claims, ‘unlawful’ enemies ultimately demarcates the boundary of international society.