The captured Somali pirate involved in the Maersk Alabama attack arrived Tuesday in New York, where he will face trial. But several newspapers have quoted a civil rights lawyer named Ron Kuby, who wonders whether the pirate was captured illegally: "I think there's a grave question as to whether America was in violation of principles of truce in warfare on the high seas. … This man seemed to come on to the [USS] Bainbridge under a flag of truce to negotiate." What's a high-seas truce?
It's the same as a truce on land. Soldiers seeking reprieve have been waving white flags since at least the Eastern Han Dynasty (25 to 220 A.D.) and, in the West, the Second Battle of Cremona (69 A.D.), fought in what is now northern Italy. The practice is now enshrined in international law and applies just as well to battles at sea as it does in other forms of warfare. Under the Hague Regulations—among the first formal statements of the laws of war—and according to U.S. naval procedures (PDF), a lawful combatant can use a white truce flag to surrender or to indicate that he wants to chat. The opposing side doesn't have to receive the flag-bearer, but if it does, the envoy is considered inviolable from attack or from being taken prisoner. That is, unless the envoy takes advantage of his position to obtain secret information, in which case he may be treated as a spy.
According to the Geneva Conventions, feigning "an intent to negotiate under a flag of truce or of surrender" is prohibited. (It's considered perfidy, or unlawful deception.) So you can't raise a white flag and then shoot your enemy; and you can't pretend you're willing to negotiate with a flag-waving envoy only to lock him up. Even if the envoy isn't literally bearing a white flag, it would be perfidy to promise immunity negotiations and then change the rules.
These protections, however, almost certainly don't apply to the captured pirate. First of all, it's unclear whether the suspect went onboard the Navy ship to engage in official negotiations or to receive medical attention for his injuries. Second, pirates aren't privileged combatants—they don't represent a state in an international armed conflict. Rather, they're criminals, and—outside of the court system—criminals can't negotiate terms. (They may have a claim to other rights, like due process and humane treatment.) A bank robber, for example, can't call a timeout, chat with the FBI hostage negotiator, and then expect to go free.
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Explainer thanks Ron Kuby, Martin Murphy of the Center for Strategic and Budgetary Assessments, Whitney Smith of the Flag Research Center, and John Fabian Witt of Columbia Law School.