Hostis humani generis
Hostis humani generis is a legal phrase meaning "enemy of all mankind", a term more of custom than formal definition. The term may have first been used by the Roman statesman Cicero, "Pirata est hostis humani generis", “a pirate is the common enemy of humankind.”  Historically, it applied to persons whose acts threatened all societies and took them outside national jurisdiction, such as pirates and slavers. After the Second World War, application of the label was extended to perpetrators of genocide, and now general usage includes transnational terrorists.
Persons suspected of complicity in these acts fell under early concepts of universal jurisdiction. Before slavery was generally accepted as wrong, piracy was condemned, and it was standard practice for any navy to apprehend, and sometimes summarily try and execute, pirates.
The exception for privateering was removed by an annex to the Declaration of Paris (1856) that ended the Crimean War; Britain and France, at the start of that war, had renounced privateering. While not all seafaring nations ratified it, it became de facto customary international law.
UNCLOS also addresses slavery. 
See main article Genocide.
McMillan cites the British jurist, Sir William Blackstone, who said "Given that a pirate, has renounced all the benefits of society and government, and has reduced himself afresh to the savage state of nature, by declaring war against all mankind, all mankind must declare war against him.” He argues that the transnational terrorist has placed himself in a similar role.
- Joseph McMillan (November 2004), "Apocalyptic Terrorism: The Case for Preventive Action", Strategic Forum, Institute for National Strategic Studies, National Defense University (no. 212)
- Division for Ocean Affairs and the Law of the Sea, Office of Legal Affairs, United Nations, Part VII, High Seas, United Nations Convention on the Law of the Sea of 10 December 1982, Article 101
- UNCLOS, Article 99
- Commentaries on the Laws of England, IV.5.iii, 72, cited by McMillan, p. 2