Hostis humani generis

Hostis humani generis (Latin for "enemy of mankind") is a legal term of art that originates in admiralty law. Before the adoption of public international law, pirates and slavers were already held to be beyond legal protection and so could be dealt with by any nation, even one that had not been directly attacked.

A comparison can be made between this concept and the common law "writ of outlawry", which declared a person outside the King's law, a literal out-law, subject to the violence of anyone. The ancient Roman civil law concept of proscription, and the status of homo sacer conveyed by proscription may also be similar.

International waters (also called "the high seas") have their own customs and usage, rules and articles, and laws. Unlike the case with land, above the high-tide mark, where title, ownership, and sovereignty are created by law based around use and possession, no nation may claim as its territory the high seas, for continuous use and possession of them is impossible; as such, no nation may thus forbid trespass through the high seas. The high seas, since they cannot be owned by anyone, are held to belong to all humanity, and every nation is held to have a separate and equal right to have its ships navigate over them; this is the concept of mare liberum, or the freedom of the seas. As the sea is the common property of all, the perils of the sea and of navigation are shared by all mariners, and all nations. A law of amity and reciprocity holds among the seafaring powers, especially in regard to matters related to the protection of life and to a lesser extent, property; for instance, the law stipulates the obligation of every mariner to assist those who are shipwrecked, and the obligation of every harbormaster to provide to any vessel in need during a storm, regardless of the flag it flies.

...
Wikipedia