Universal jurisdiction

Universal jurisdiction is a concept in international law, in which certain offenses are considered sufficiently grave that any Requesting State (i.e., nation) may apprehend, or ask for extradition of, an individual who has been charged, by a second state. In the most general form, there is no requirement that the defendant be a national of the Requesting State, that the the victim(s) be a citizen of the Requesting State, or, indeed, that there are  any links to the Requesting State.

International law
Some of its legal framework relates to the legal doctrine of hostis humani generis, most commonly applied to pirates or slavers, who could be taken into custody by any navy. While the International Military Tribunal (Nuremberg) and International Military Tribunal (Tokyo) were not established by treaty, and indeed prosecuted and executed defendants based on ex post facto rules of criminality, those Tribunals are treated with a good deal of legitimacy if not universally accepted precedent.

The Geneva Conventions call for prosecution of offenders by any concerned state, but do not establish a structure for trying individuals.

More contemporary usage ties to the Rome Statute establishing the International Criminal Court; not all nations, including a number of major ones, have ratified the ICC treaty.

The Rome Statute specifies a "State Party which has received a request for provisional arrest or for arrest and surrender shall immediately take steps to arrest the person in question in accordance with its laws and theprovisions of [the Rome Statute].

United States
In the 1984 case of Filartiga v. Pena-Irala, the U.S. Second Circuit Court of Appeals, held that torturers fell under universal jurisdiction, and, further, that the doctrine of command responsibility applied to the superiors of persons actually committing torture. In the 1987 case of Forti v. Suarez Mason‎, the US Circuit Court for the Northern District of California held that torturers fell under universal jurisdiction.

The United States has not ratified the Rome Statute and is not an ICC participant.

Spain
Perhaps the best-known recent case dealt with the 1998 Spanish request that the United Kingdom apprehend Augusto Pinochet for offenses committed in Chile. This was based on Spain's 1985 Organic Law that defined the jurisdiction of Spanish courts over the obvious case when the criminal act took place on Spanish soil, the less obvious case when alleged crime was committed by a Spaniard outside Spain, or when the crime is of international concern, such as genocide, piracy and terrorism.

Spain has been an activist country in pursuing individuals believed to have committed relevant crimes, but it is considering legislation, called the Reform of the Judiciary Act, reducing its scope to situations where the crimes must have been committed against Spaniards, or at least have some historic link to Spain. In the past, Spanish courts have considered actions related to alleged crimes in Rwanda, Tibet, Guatemala, and China. The reports also suggest that the arrests must be made in Spain; extradition will not be used. The proposed legislation may have been triggered by a Spanish judge, Fernando Andreu, ordering an investigation into whether an Israeli actions in the Gaza Strip constituted a crime against humanity.