Hostis humani generis

Hostis humani generis is a legal phrase meaning "enemy of all mankind", a term more of custom than formal definition. Historically, it applied to persons whose acts both threatened all societies, and also took them outside national jurisdiction, such as piracy and slavery. After the Second World War, genocide was usually considered within its scope, and transnational terrorism is now part of the general usage.

Piracy
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.

Piracy is now covered by the United Nations Convention on the Law of the Sea.