A flaw in most discussions of "piracy" is the lack of a coherent and historically accurate definition of what the term meant to the Framers when they wrote the Piracy Clause. The United States Supreme Court decision in the 1820 case of United States v. Smith, 18 US 153, did us a great disservice when it confined it to "robbery at sea", following the comment by Framer James Wilson in one of his lectures. "Piracy" as originally understood included more than robbery and more than "at sea".
As a legal term, as distinct from a lay term, the term is best understood as "a warlike act by a nonstate actor against assets of a country other than his own". If it were against assets of his own country it would be treason or an ordinary felony. Thus it includes actions for political purposes as well as for gain, and on land as well as at sea or on other waters.
Thus, all the acts now classified as "terrorism" would be species of "piracy" in constitutional terms, and the Piracy Clause in the U.S. Constitution is the only authority for prosecuting them if they are foreign nationals. The Treason Clause is the only authority for prosecuting them if they are U.S. nationals.
An important distinction must be understood. "Terrorism" (irjaf, or sometimes irhab or hirabah in Arabic) is not a crime (felony). It is also not war (bellum). It is guerra, "war against all mankind, or an "offense against the law of nations"). Neither the standards for due process for felony, nor for bellum (armed conflict) against a legitimus hostis apply. The only due process needed to try a latrunculus (latro) before a military tribunal is to establish the fact that the accused is a latro. Once that is established, no further argument is needed. The sentence is death, which may be carried out immediately.
Originally the standard of due process for piracy or brigandage (latrocinium) was a military tribunal on the spot, followed by immediate execution. Because it is a crime of "universal jurisdiction", there is no issue of double jeopardy, and the same offenses can be tried by any number of nations under their national laws against piracy. The standard penalty has always been death, to be deferred only long enough to obtain intelligence on other pirating operations and offenders.
This distinction was first made by the Romans and subsequently incorporated into international law by way of medieval European jurisprudence. As the eminent military historian, Sir Michael Howard, wrote in right after 9/11, the Romans distinguished between bellum, war against legitimus hostis, a legitimate enemy, and guerra, war against latrunculi – pirates, robbers, brigands, and outlaws – ”the common enemies of mankind.” The former, bellum, became the standard for interstate conflict. It is here for instance that the Geneva Conventions were meant to apply. They do not apply to the latter, Guerra – indeed, punishment for latrunculi traditionally has been summary execution. While not employing the term, many legal experts agree that al Qaeda fighters are latrunculi – hardly distinguishable by their actions from pirates and the like. Who knows what some silly judge might rule in the future, but at least so far, no terrorist organization has been deemed a combatant under the laws of armed conflict.