A professor of piracy often deals with eye-patch and hook jokes. Many people who find this academic specialty intriguing lose interests when they learn that modern pirates wear jeans, tee-shirts and flip-flops, or when they're feeling natty, fatigues. They certainly don't fly a black flag. They have very bad personal hygiene: forget Johnny Depp and Cary Elwes.

Yet the ordinary appearance of pirates leads to a potentially serious problem in prosecuting them.

Universal jurisdiction only applies to pirates. Captured Somalis are likely to insist in court that they are not pirates but rather simple fishermen, erroneously seized by a foreign navy. What makes the claim compelling is that most pirates are in fact fishermen. Piracy is not a full-time job. Simply having weapons on a boat would not distinguish the pirates from many other Somalis. Establishing the very identity or even nationality of captured individuals will be difficult, as they are unlikely to possess identification. (This will also make it hard to know whether a captured pirate is a minor; or even what nation he comes from, making consular rights and other issues quite difficult to administer.)

Such challenges must be taken seriously, because the alternative is the detention of innocent civilians. To be sure, treating the detainees as civilians would require giving credence to some dubious factual claims. However, the same is true of many Guantanamo detainees captured in Afghanistan and elsewhere. They have claimed that they were innocent passers by, aid workers, tourists, minors, or simply ignorant of the nature and aims of the organization for which they worked. Regardless of their plausibility, these claims won significant sympathy for the detainees. Moreover, U.S. courts have held that because the power to detain depends on the foreigner's status as a combatant, detainees can appear before tribunals to challenge the factual basis for being classified as a combatant even before a full trial for their alleged crimes.

Quite simply, making a criminal case against armed foreigners seized in remote parts of the world is very difficult. These concerns are not speculative. Evidentiary problems have already forced the U.S. Navy to release most of the pirates it seized in the wake of its January 2009 agreement with Kenya. Even though they were caught in response to a distress call from a commercial vessel, the evidence was "not ironclad."